Optical device, and virtual image display

ABSTRACT

A virtual image display device with an optical waveguide to guide, by internal total reflection, parallel pencil groups meeting a condition of internal total reflection, a first reflection volume hologram grating to diffract and reflect the parallel pencil groups incident upon the optical waveguide from outside and traveling in different directions as they are so as to meet the condition of internal total reflection inside the optical waveguide and a second reflection volume hologram grating to project the parallel pencil groups guided by internal total reflection inside the optical waveguide as they are from the optical waveguide by diffraction and reflection thereof so as to depart from the condition of internal total reflection inside the optical waveguide.

RELATED APPLICATION DATA

This application is a continuation of U.S. patent application Ser. No.13/453,071 filed Apr. 23, 2012, which is a continuation of U.S. patentapplication Ser. No. 13/279,997, filed Oct. 24, 2011, now issued as U.S.Pat. No. 8,213,755, which is a continuation of U.S. patent applicationSer. No. 13/102,397, filed May 6, 2011, now issued as U.S. Pat. No.8,073,296, which is a continuation of U.S. patent application Ser. No.12/778,290, filed May 12, 2010, now issued as U.S. Pat. No. 8,023,783,which is a division of U.S. patent application Ser. No. 12/134,846,filed Jun. 6, 2008, now issued as U.S. Pat. No. 7,747,113, which is adivision of U.S. patent application Ser. No. 10/555,180, filed Oct. 31,2005, now issued as U.S. Pat. No. 7,418,170, the entirety all of whichare incorporated herein by reference to the extent permitted by law.U.S. patent application Ser. No. 10/555,180 is the Section 371 NationalStage of PCT/JP2005/005761. This application claims the benefit ofpriority to PCT International Application No. PCT/JP2005/005761, filedMar. 28, 2005 and Japanese Patent Application No. 2004-097222 filed onMar. 29, 2004.

BACKGROUND

The present invention generally relates to a virtual image displaydevice for displaying a two-dimensional image enlarged through a virtualimage optical system for viewing by an viewer, and more particularly toa slim optical device using a hologram optical element, especially, areflection volume hologram grating, to guide an image light to bedisplayed to the pupil of the viewer and a virtual image display deviceusing the optical device.

For viewing of a virtual image in a magnified form by a viewer, avirtual image viewing optical system as shown in FIG. 1 has beenproposed.

In the virtual image viewing optical system shown in FIG. 1, an imagelight displayed on an image display device 301 is first incident upon anoptical waveguide 302 having a transmission hologram lens 303 providedtherein. While being formed by the transmission hologram lens 303 into aparallel light, the incident image light is deflected at an angle fortotal reflection inside the optical waveguide 302.

The optical waveguide 302 has also a transmission hologram grating 304provided therein in line with the transmission hologram lens 303 at apredetermined distance from the latter. After traveling through theoptical waveguide 302 while being totally reflected, the image light isincident upon the transmission hologram grating 304 by which the imagelight is diffracted again and projected in the parallel-light state tooutside the optical waveguide 302 and toward the viewer's pupil.

For viewing of a virtual image in a magnified form by a viewer, therehas also been proposed a virtual image viewing optical system as shownin FIG. 2.

In the virtual image viewing optical system shown in FIG. 2, an imagelight displayed on an image display element 401 is directed forincidence upon an optical waveguide 403 through a free-form surfacedprism 402. As shown in FIG. 3, the optical waveguide 403 includes afirst HOE (Holographic Optical Element) 404 and second HOE 405 providedin an incident region Z1 at the incident side of the optical waveguide403, and a third HOE 405 and fourth HOE 406 provided in an outgoingregion Z2 at the outgoing side. The image light incident upon theoptical waveguide 403 is continuously diffracted and reflected at thelight-incident side of the optical waveguide 403, first HOE 404 providedon a surface opposite to the light-incident side and at the second HOE405 provided at the light-incident side, and deflected inside theoptical waveguide 304 to go at a larger angle than a critical angle fortotal reflection. More specifically, image light L1 incident upon theoptical waveguide 403 is diffracted and reflected at a firstincident-side diffraction-reflecting surface D1 of the first HOE 404 andthen at a second incident-side diffraction-reflecting surface D2 of thesecond HOE 405 to go at a larger angle α₂ than the critical angle. Itshould be noted that when the image light L1 is diffracted and reflectedat the first diffraction-reflecting surface D1, it will go at a smallerangle α₁ than the critical angle.

The image light L2 directed at the larger angle than the critical angleinside the optical waveguide 403 travels while being totally reflectedinside the optical waveguide 403, and is then continuously diffractedand reflected at a first outgoing-side diffraction-reflecting surface D3of a fourth HOE 407 and then at a second outgoing-sidediffraction-reflecting surface D4 of a third HOE 406 to go at a smallerangle α₃ than the critical angle and outgo toward the optical pupil ofthe viewer outside the optical waveguide 403.

However, the virtual image viewing optical system shown in FIG. 1 isdisadvantageous as will be described below:

Firstly, in the virtual image viewing optical system shown in FIG. 1,divergent light projected from an image display device 301 is incidentdirectly upon the transmission hologram lens 303 in the opticalwaveguide 302. When the distance between the image display device 301and transmission hologram lens 303 is increased, namely, when the focaldistance of the transmission hologram lens 303 is increased, for anincreased magnification of the optical system, the diameter of the pupil305 cannot be increased because the transmission hologram lens 303 hasonly a relatively small diffraction acceptance angle.

Secondly, since the interference fringe of the transmission hologramlens 303 has a complicated structure having a spherical phase component,it is difficult to combine or laminate the interference fringes togetherfor a larger diffraction acceptance angle and the lens 303 cannot bedesigned to diffract light rays equal in wavelength and incident angleto each other with different efficiencies at the same diffraction angle.

Thirdly, in the virtual image viewing optical system shown in FIG. 1,since the transmission hologram lens 303 provided on the opticalwaveguide 302 diffracts image light rays coming from the image displaydevice 301 while forming the light rays into a parallel pencil of rays,that is, while generating an optical power, a large monochromaticeccentric aberration will be caused, which will also lead to a reducedresolution of an image projected on the pupil.

Fourthly, the virtual image viewing optical system shown in FIG. 1 usesthe transmission hologram grating 304 to correct achromatic aberrationoccurring in the transmission hologram lens 303. Since light raysincident upon the transmission hologram grating 304 is deflected only inthe direction in the plane of the drawing in FIG. 1, aberrationoccurring in a direction perpendicular to at least the drawing planecannot be canceled. The diffraction-caused chromatic aberration takesplace because the two transmission holograms (transmission hologram lens303 and transmission hologram grating 304) provided in the opticalwaveguide 302 are different from each other and there can be usedsubstantially only a light source of which the waveband is narrow, whichis a large constraint to this conventional virtual image viewing opticalsystem.

A simulation was actually made by retracing the light incident upon thepupil in the virtual image viewing optical system shown in FIG. 1. Theresult of simulation shows that even when the chromatic aberration wascorrected by the two transmission holograms, it was found that awavelength shift of ±2 nm resulted in a shift of ±30 μm of image lighton the image display device 301.

If the two transmission holograms are identical transmission volumehologram gratings having no optical power, for example, another problemdescribed below will take place.

It is well known that at a constant incident angle, the diffractionacceptance waveband of the transmission volume hologram is broader thanthat of the reflection volume hologram. Therefore, incase the wavebandof a light source is broad or in case the wavelength interval of a lightsource for each of RGB (R: Red light; G: Green light; B: Blue light)that are three primary colors of light is narrow (in case the wavebandof each color light is broad), chromatic dispersion due to vastdiffraction, that is, diffraction chromatic dispersion will take place.

Even a transmission volume hologram prepared for green (of 550 nm incentral wavelength), for example, has a diffraction efficiency of about10% with a waveband of 400 to 630 nm and will partially diffract lightfrom a blue LED (Light Emitting Diode) (of 410 to 490 nm inlight-emitting wavelength) and light from a red LED (of 600 to 660 nm inlight-emitting wavelength).

The chromatic aberration due to the diffraction chromatic dispersion canbe canceled by two holograms equal in grating pitch to each other.However, in case the chromatic dispersion made by one of the hologramsis larger, a pencil of rays traveling inside an optical waveguide willspread largely, resulting in a following problem. When the largelyspread pencil of rays having been diffracted by the first hologram andtraveled inside the optical waveguide is diffracted at the secondhologram and projected from the optical waveguide, it will spreadlargely in the traveling direction on the basis of its wavelength andlead to a deteriorated color uniformity of a virtual image on theviewer's pupil.

On the other hand, in the reflection volume hologram, the diffractionacceptance waveband one interference fringe has is narrow. Therefore, incase image light is colored, the colors (total reflection angle insidethe optical waveguide) can be equalized in diffraction angle bylaminating hologram layers together for each of RGB or combining theinterference fringe of each of RGB.

On the contrary, with a constant incident wavelength, the diffractionacceptance angle of the transmission volume hologram is smaller thanthat of the reflection volume hologram and thus it will be difficult toincrease the diameter of the pupil 305 or field angle.

Also, since in the virtual image viewing optical system shown in FIGS. 2and 3, an image of the image display element 401 is intermediate-formedinside the optical waveguide 403, the first HOE 404, second HOE 405,third HOE 406 and fourth HOE 407 should have an optical power in aneccentric layout. Therefore, also in this virtual image viewing opticalsystem, eccentric aberration will occur as in the virtual image viewingoptical system shown in FIG. 1.

In the virtual image viewing optical system shown in FIGS. 2 and 3, thefree-form surfaced prism 402, first HOE 404, second HOE 405, third HOE406 and fourth HOE 407 are provided axial-symmetrically with respect toeach other to reduce the eccentric aberration. However, since the upperlimit of the diffraction efficiency of each HOE is substantially 70 to80%, the total of the diffraction efficiency of the four HOEs is thefourth power of 70 to 80% and thus the diffraction efficiency will beconsiderably lower.

As above, in a hologram having a complicated interference patter, it isdifficult to increase the diffraction acceptance of the interferencefringe by laminating hologram layers together or combining theinterference fringe. Therefore, the pupil diameter cannot be increased.

Also, since convergent light (down to intermediate image formation) ordivergent light (after the intermediate image formation) travels insidethe optical waveguide 403, a pencil of rays not diffracted by the firstreflection and diffraction but totally reflected again in the plane ofthe optical waveguide 403 cannot be used a any image display light orimage light any longer. Therefore, the conventional virtual imageviewing optical system can neither use light with any improvedefficiency nor enlarge the viewable range.

SUMMARY OF THE INVENTION

Accordingly, the present invention has an object to overcome theabove-mentioned drawbacks of the related art by providing an opticaldevice and virtual image viewing optical system in which the imageresolution is increased by eliminating or reducing monochromaticaberration and diffraction chromatic aberration, light is diffractedwith a higher efficiency by reducing the number of hologram elementsused, color of display image is uniformized and pupil diameter isincreased.

The above object can be attained by providing an optical deviceincluding according to the present invention:

an optical waveguide to guide groups of pencils of rays meeting acondition of internal total reflection inside the optical waveguide byinternal total reflection of the parallel pencil groups;

a first reflection volume hologram grating to diffract and reflect theparallel pencil groups incident upon the optical waveguide from outsideand traveling in different directions as they are so as to meet thecondition of internal total reflection inside the optical waveguide; and

a second reflection volume hologram grating to project the parallelpencil groups guided by internal total reflection inside the opticalwaveguide as they are from the optical waveguide by diffraction andreflection thereof so as to depart from the condition of internal totalreflection inside the optical waveguide,

some parallel pencils of the parallel pencil groups guided through theoptical waveguide being totally reflected different numbers of times fora period from external incidence upon the optical waveguide untiloutgoing from the optical waveguide.

Also the above object can be attained by providing a virtual imageviewing optical system including according to the present invention:

an image display element;

a collimating optical system to form a pencil of rays coming from eachpixel of the image display element into parallel pencil groups travelingin different directions;

an optical waveguide to guide, by internal total reflection, parallelpencil groups meeting a condition of internal total reflection insidethe optical waveguide;

a first reflection volume hologram grating to diffract and reflect theparallel pencil groups incident upon the optical waveguide from outsideand traveling in different directions as they are so as to meet thecondition of internal total reflection inside the optical waveguide; and

a second reflection volume hologram grating to project the parallelpencil groups guided by internal total reflection inside the opticalwaveguide as they are from the optical waveguide by diffraction andreflection thereof so as to depart from the condition of internal totalreflection inside the optical waveguide,

some parallel pencils of the parallel pencil groups guided through theoptical waveguide being totally reflected different numbers of times fora period from external incidence upon the optical waveguide untiloutgoing from the optical waveguide.

Also the above object can be attained by providing a virtual imageviewing optical system including according to the present invention:

a light source to emit a pencil of rays;

a collimating optical system to form the pencil of rays from the lightsource into a parallel pencil;

a scanning optical system to form, by horizontal and vertical scan, theparallel pencil into parallel pencil groups traveling in differentdirections;

an optical waveguide to guide, by internal total reflection, theparallel pencil groups meeting a condition of internal total reflectioninside the optical waveguide;

a first reflection volume hologram grating to diffract and reflect theparallel pencil groups incident upon the optical waveguide from thecanning optical system and traveling in different directions as they areso as to meet the condition of internal total reflection inside theoptical waveguide; and

a second reflection volume hologram grating to project the parallelpencil groups guided by internal total reflection inside the opticalwaveguide as they are from the optical waveguide by diffraction andreflection thereof so as to depart from the condition of internal totalreflection inside the optical waveguide,

some parallel pencils of the parallel pencil groups guided through theoptical waveguide being totally reflected different number of times fora period from external incidence upon the optical waveguide untiloutgoing from the optical waveguide.

According to the present invention, parallel pencil groups incident uponthe optical waveguide from outside and traveling in different directionsis diffracted and reflected by the first reflection volume hologramgrating as they are so as to meet the condition of internal totalreflection inside the optical waveguide, and the parallel pencil groupsguided by total reflection inside the optical waveguide is projected bydiffraction and reflection by the second reflection volume hologramgrating as they are from the optical waveguide so as to depart from thecondition of internal total reflection inside the optical waveguide.

At this time, since some parallel pencils of the parallel pencil groupsguided by internal total reflection inside the optical waveguide areincident upon the optical waveguide from outside and totally reflecteddifferent number of times for a period until outgoing from the opticalwaveguide, the optical waveguide can be formed very thin andsufficiently large along the length thereof.

Therefore, the virtual image optical device according to the presentinvention can be designed more lightweight and compact, and can beproduced with less costs. Also, in case this virtual image opticaldevice is used as an HMD (Head Mounted Display), possible discomfortgiven to the user when it is mounted on the user's head can considerablybe reduced.

Also, according to the present invention, diffraction can be made withan improved efficiency since only two reflection volume hologramgratings, first and second, are used. Further, since the reflectionvolume hologram grating is smaller in diffraction acceptance wavelengthand larger in diffraction acceptance angle than the transmission volumehologram grating, so a display image can be uniformized in colors andthe pupil diameter can be increased.

Further, since the first and second reflection volume hologram gratingsused in the present invention do not act as any lens, so themonochromatic eccentric aberration can be eliminated. Also, since thediffraction acceptance wavelength is small and thus the diffractionchromatic aberration can be reduced, an image of a high resolution canbe displayed on the viewer's pupil.

Moreover, according to the present invention, the interference fringerecorded on the first reflection volume hologram grating is equal inpitch on the hologram surface to that recorded on the second reflectionvolume hologram grating. Therefore, parallel rays of light incident atthe same wavelength and incident angle can be prevented from beingdiffracted and reflected at different angles of diffraction and avirtual image of a high resolution can be displayed on the viewer'spupil.

Also, since the interference fringe recorded on each of the first andsecond reflection volume hologram gratings used in the present inventionis a simple diffraction grating in which the interference fringe issimple, so the interference fringes can easily be combined together andthe hologram layers each having the interference fringe recorded thereoncan be laminated together. Thus, it is possible to diffract and reflectparallel pencil groups different in wavelength from each other, forexample, RGB (R: Red light G: Green light; B: Blue light) as threeprimary colors of light, with an increased diffraction acceptance angleand without occurrence of diffraction chromatic aberration and reductionof color gamut.

These objects and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription of the best mode for carrying out the present invention whentaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation of a conventional virtual image viewingoptical system.

FIG. 2 is also a side elevation of another conventional virtual imageviewing optical system.

FIG. 3 is a side elevation of an optical waveguide included in theconventional virtual image viewing optical system shown in FIG. 2.

FIG. 4 shows a diffraction efficiency distribution of a transmissionvolume hologram grating.

FIG. 5 shows a diffraction efficiency distribution of a reflectionvolume hologram grating.

FIG. 6 shows the dependency on incident angle of the diffractionefficiency of the transmission and reflection volume hologram gratings.

FIG. 7 shows the dependency on incident wavelength of the diffractionefficiency of the transmission and reflection volume hologram gratings.

FIG. 8 is a side elevation of a virtual image display device as a firstembodiment of the present invention.

FIG. 9 is a side elevation of a first reflection volume hologram gratingincluded in the image display device in FIG. 8.

FIG. 10 is a side elevation of a second reflection volume hologramgrating included in the image display device in FIG. 8.

FIG. 11 is a side elevation of a virtual image display device as asecond embodiment of the present invention.

FIG. 12 is a side elevation of a first reflection volume hologramgrating included in the image display device in FIG. 11.

FIG. 13 is a side elevation of a second reflection volume hologramgrating included in the image display device in FIG. 11.

FIG. 14 is a side elevation of a virtual image display device as a thirdembodiment of the present invention.

FIG. 15 is a side elevation of a first reflection volume hologramgrating included in the image display device in FIG. 14.

FIG. 16 is a side elevation of a first reflection volume hologramgrating included in the image display device in FIG. 14.

FIG. 17 is a side elevation of a virtual image display device as afourth embodiment of the present invention.

FIG. 18 is a side elevation of a first reflection volume hologramgrating included in the image display device in FIG. 17.

FIG. 19 is a side elevation of a second reflection volume hologramgrating included in the image display device in FIG. 17.

FIG. 20 is a side elevation of a variant of the second reflection volumehologram grating included in the image display device in FIG. 17.

FIG. 21 is a side elevation of a virtual image display device as a fifthembodiment of the present invention.

FIG. 22 is a side elevation of a virtual image display device as a sixthembodiment of the present invention.

FIG. 23 is a side elevation of the second reflection volume hologramgrating included in the virtual image display device in FIG. 22, showingthe diffraction and reflection by the grating.

FIG. 24 is a side elevation of the second reflection volume hologramgrating included in the virtual image display device in FIG. 22.

FIG. 25 is a side elevation of one of hologram layers included in thesecond reflection volume hologram grating in FIG. 22.

FIG. 26 is a side elevation of a variant of the hologram in the secondreflection volume hologram grating in FIG. 22.

FIG. 27 explains the relation between the slant angle of an interferencefringe recorded on the second reflection volume hologram grating in FIG.22 and incident angle of incident parallel pencils.

FIG. 28 shows a variation of the slant angle of an interference fringefor a maximum diffraction efficiency when a parallel pencil is incidentat a different angle upon the second reflection volume hologram gratingin FIG. 22.

FIG. 29 is a side elevation of an image display device as a seventhembodiment of the present invention.

FIG. 30 is a side elevation of an image display device as an eighthembodiment of the present invention.

FIG. 31 is a side elevation of an image display device as a ninthembodiment of the present invention.

FIG. 32 is a side elevation of an image display device as a tenthembodiment of the present invention.

FIG. 33 is a side elevation of an image display device as an eleventhembodiment of the present invention.

FIG. 34 is a side elevation of an image display device as a twelfthembodiment of the present invention.

FIG. 35 is a side elevation of an image display device as a thirteenthembodiment of the present invention.

FIG. 36 is a side elevation of an image display device as a fourteenthembodiment of the present invention.

FIG. 37 is a side elevation of an image display device as a fifteenthembodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The present invention will be described in detail below concerning anoptical device and virtual image display device as the embodimentsthereof with reference to the accompanying drawings.

Prior to description of the embodiments of the present invention, thecharacteristics of the reflection volume hologram grating used in eachof the embodiments will be described in comparison with those of thetransmission volume hologram grating used in the conventional opticaldevices and virtual image display devices.

As having previously been described concerning the conventionaltechnology, the reflection volume hologram grating is smaller indiffraction acceptance waveband and diffraction acceptance angle thanthe transmission volume hologram grating.

The above will be described in detail herebelow with reference to FIGS.4 to 7. FIG. 4 shows the diffraction efficiency distribution of thetransmission volume hologram grating that transmits, by diffraction,vertically incident parallel pencils of 550 nm in wavelength at an angleof 45 deg. in a medium of 1.52 in refractive index, and FIG. 5 shows thediffraction efficiency distribution of the reflection volume hologramgrating that diffracts and reflects parallel pencil groups (of 400 to700 nm in wavelength) incident at an angle of vertical incidence ±5 deg.at an angle of 45 deg. in a medium of 1.52 in refractive index.

In FIGS. 4 and 5, the hatched portion indicates a region defined by anincident wavelength and incident angle that assure the diffractionefficiency. The refractive index modulation of each hologram wasselected to be 0.05, and a thickness of the hologram layer was selectedfor a peak diffraction efficiency of 99% or more.

As seen from the diffraction efficiency distributions of thetransmission and reflection volume hologram gratings shown in FIGS. 4and 5, respectively, the variation of a wavelength that can bediffracted by the reflection volume hologram grating varies less thanthat by the transmission volume hologram grating in the same range ofincident angle or the diffraction acceptance angle in the reflectionvolume hologram grating is larger than that in the transmission volumehologram grating in the same range of incident wavelength.

FIGS. 6 and 7 show, in other forms, the diffraction efficiencydistributions shown in FIGS. 4 and 5. FIG. 6 shows the dependency onincident angle (incident wavelength of 550 nm) of the diffractionefficiency of the transmission and reflection volume hologram gratings.It should be noted that in FIG. 6, the solid line indicates thedependency on incident angle of the transmission volume hologram gratingwhile the dashed line indicates the dependency on incident angle of thereflection volume hologram grating. As apparent from FIG. 6, thereflection volume hologram grating is larger in diffraction acceptanceangle than the transmission volume hologram grating.

Also, FIG. 7 shows the dependency on incident wavelength (incident angleof 0 deg.) of the diffraction efficiency of the transmission andreflection volume hologram gratings. It should be noted that in FIG. 7,the solid line indicates the dependency on incident wavelength of thetransmission volume hologram grating while the dashed line indicates thedependency on incident wavelength of the reflection volume hologramgrating. As apparent from FIG. 7, the reflection volume hologram gratingis larger in diffraction acceptance wavelength than the transmissionvolume hologram grating.

Based on the general characteristics of the reflection volume hologramgrating, there will be explained the first to sixth embodiments givenherein as the best modes for carrying out the present invention.

First Embodiment

FIG. 8 shows a virtual image display device as a first embodiment of thepresent invention. The virtual image display device is generallyindicated with a reference numeral 10. The virtual image display device10 includes an image display element 11 to display an image, and avirtual image optical system to guide incident display light from theimage display element 11 to a pupil 16 of the viewer.

The image display element 11 is for example an organic EL (ElectroLuminescence) display, inorganic EL display, liquid crystal display(LCD) or the like.

The virtual image optical system includes a collimating optical system12, optical waveguide 13, and a first reflection volume hologram grating14 and second reflection volume hologram grating 15 provided on theoptical waveguide 13.

The collimating optical system 12 receives an incident pencil from eachpixel of the image display element 11 and forms the pencils intoparallel pencil groups different in angle of field from each other. Theparallel pencil groups projected from the collimating optical system 12and different in angle of field from each other are incident upon theoptical waveguide 13.

The optical waveguide 13 is a slim, parallel, flat optical waveguideincluding mainly an optical surface 13 a having provided at one endthereof a light-incident port 13 a 1 upon which there are incidentparallel pencil groups projected from the collimating optical system 12and different in angle of field from each other and at the other end alight-outgoing port 13 a 2 from which the light is projected, and anoptical surface 13 b opposite to the optical surface 13 a.

On the optical surface 13 b of the optical waveguide 13, there areprovided the first reflection volume hologram grating 14 in a positionwhere it is opposite to the light-incident port 13 a 1 at the opticalsurface 13 a and the second reflection volume hologram grating 15 in aposition where it is opposite to the light-outgoing port 13 a 2 at theoptical surface 13 a.

FIGS. 9 and 10 show the first and second reflection volume hologramgratings 14 and 15 each having interference fringes recorded thereon. Asshown in FIGS. 9 and 10, the first and second reflection volume hologramgratings 14 and 15 have recorded on hologram surfaces 14S and 15Sthereof, respectively, three types of interference fringes different inslant angle that is an angle of the interference fringes slanting fromeach other. A combination of the three types of interference fringes arerecorded with the same pitch on the hologram surfaces 14S and 15S,respectively. Each of the first and second reflection volume hologramgratings 14 and 15 is a monochromatic hologram grating of about 20 nm indiffraction acceptance waveband and has the diffraction acceptance anglethereof increased by recording the three types of interference fringesdifference in slant angle from each other as above.

As shown in FIG. 9, the reflection volume hologram grating 14 has aplurality of interference fringes 14 a, 14 b and 14 c recorded thereonat slant angles θa, θb and θc, respectively, with the same pitchindependently of the locations of the interference fringes. Similarly,the second reflection volume hologram grating 15 shown in FIG. 10 has aplurality of interference fringes 15 a, 15 b and 15 c at slant anglesθa, θb and θc, respectively, with the same pitch. Therefore, the firstand second reflection volume hologram gratings 14 and 15 have theirinterference fringes disposed on the optical surface 13 b of the opticalwaveguide 13 to be symmetrical with respect to a plane perpendicular tothe optical surface 13 b.

The parallel pencil groups incident upon the light-incident port 13 a 1of the optical waveguide 13 and different in angle of field from eachother are incident upon the above-mentioned first reflection volumehologram grating 14 and diffracted and reflected as they are. Theparallel pencil groups thus diffracted and reflected will travel whilebeing repeatedly totally reflected between the optical surfaces 13 a and13 b of the optical waveguide 13 and will be incident upon theabove-mentioned second reflection volume hologram grating 15.

The length, and thickness between the optical surfaces 13 a and 13 b, ofthe optical waveguide 13 are set to provide such an optical path lengththat parallel light beans different in angle of field from each otherand traveling inside the optical waveguide 13 are totally reflecteddifferent numbers of times correspondingly to their angles of fielduntil they arrive at the second reflection volume hologram grating 15.

More specifically, ones of the parallel pencils in group incident uponthe optical waveguide 13 while being slanted toward the secondreflection volume hologram grating 15, that is, parallel pencilsincident at a larger angle, are reflected a smaller number of times thanparallel pencils incident upon the optical waveguide 13 while not beingslanted toward the second reflection volume hologram grating 15, thatis, parallel pencils incident at a smaller angle for the reason that theparallel pencil groups incident upon the optical waveguide 13 will bedifferent in angle of field from each other. Namely, since the parallelpencils are incident upon the first reflection volume hologram grating14 at different angles and thus projected at different angles ofdiffraction, they are totally reflected at different angles. Thus, whenthe optical waveguide 13 is designed slim having a sufficient length,the parallel pencils will be reflected different numbers of times,respectively.

A group of parallel pencils different in angle of field from each otherand incident upon the second reflection volume hologram grating 15 arediffracted and reflected to depart from the condition of totalreflection, projected at the light-outgoing port 13 a 2 of the opticalwaveguide 13, and incident upon the pupil 16 of the viewer.

As above, the second reflection volume hologram grating 15 is providedon the optical surface 13 b of the optical waveguide 13 so that theinterference fringes recorded thereon take the same shape as that of theinterference fringes on the first reflecting volume hologram grating 14,rotated 180 deg. on the hologram surface. Therefore, since the parallelpencil groups to be reflected by the second reflection volume hologramgrating 15 will be reflected at an angle equal to the angle of incidenceupon the first reflecting volume hologram grating 14, a display imagewill be displayed on the pupil 16 with a high resolution withoutblurring.

Provided with the first and second reflection volume hologram gratings14 and 15 that do not act as any lens, the virtual image display device10 can eliminate and reduce monochromatic eccentric aberration anddiffraction chromatic aberration.

Note that although the first and second reflection volume hologramgratings 14 and 15 are disposed for their hologram surfaces 14S and 15Sto be parallel to the optical surface 13 b of the optical waveguide 13,the present invention is not limited to this geometry but they may bedisposed for their hologram surfaces 14S and 15S to be at apredetermined angle in relation to the optical surface 13 b.

Second Embodiment

FIG. 11 shows a virtual image display device as a second embodiment ofthe present invention. The virtual image display device is generallyindicated with a reference numeral 20. The virtual image display device20 as the second embodiment of the present invention displays a virtualimage of a color image. It should be noted that in FIG. 11, only lightrays directed at the central angle of field are illustrated mainly foreasy viewing of the drawing.

The virtual image display device 20 includes an illumination opticalsystem 30 forming a light source, and a virtual image optical system toguide incident illumination light from the illumination optical system30 to the pupil 16 of the viewer.

The illumination optical system 30 includes an LED (light-emittingdiode) light source 31R to emit red light, LED light source 31G to emitgreen light, LED light source 31B to emit blue light, and a colorsynthesizing prism 32.

Red light, green light and blue light emitted from the LED light sources31R, 31G and 31B are blended by the color synthesizing prism 32 that isa cross prism into while light, and projected to the virtual imageoptical system.

The virtual image optical system includes a collimating optical system22 to form illumination light emitted from the illumination opticalsystem 30 into a parallel pencil, rotating mirrors 21A and 21B to makespatial modulation of the parallel pencil from the collimating opticalsystem 22, optical waveguide 23 receive the illumination light havingsubjected to the spatial modulation in the rotating mirrors 21A and 21B,and a first reflection volume hologram grating 24 and second reflectionvolume hologram grating 25 provided on the optical waveguide 23.

The collimating optical system 22 forms the illumination light into aparallel pencil and emits the latter to the rotating mirror 21A thatworks as a downstream spatial modulator.

The rotating mirrors 21A and 21B function each as a spatial modulator tomake spatial modulation of the parallel pencils from the collimatingoptical system 22. As shown in FIG. 11, the rotating mirror 21A rotatesabout an axis of rotation A parallel to the plane of the drawing in thedirection of arrow A1. The rotating mirror 21B rotates about an axis ofrotation B perpendicular to the axis of rotation A and normal to theplane of the drawing in the direction of arrow B1. The rotating mirrors21A and 21B have the rotation thereof controlled by a microcomputer (notshown) correspondingly to an image to be displayed.

The parallel pencil emitted from the collimating optical system 22 tothe rotating mirror 21A is reflected toward the rotating mirror 21Bwhile being scanned by the rotating prism 21A as it is in a directionperpendicular to the plane of the drawing. The parallel pencil incidentupon the rotating mirror 21B is reflected as parallel pencil groupsdifferent in traveling direction from each other toward the opticalwaveguide 23 while being scanned as it is in a direction parallel to theplane of the drawing.

Note that specifically, the rotating mirrors 21A and 21B form together ascanning optical system that forms the parallel pencils emitted from thecollimating optical system 22 into parallel pencil groups traveling indifferent directions by scanning the parallel pencils from thecollimating optical system 22.

The optical waveguide 23 is a slim, parallel, flat optical waveguideincluding mainly an optical surface 23 a having provided at one endthereof a light-incident port 23 a 1 upon which there are incidentparallel pencil groups reflected by the rotating mirror 21B and at theother end a light-outgoing port 23 a 2 from which the light isprojected, and an optical surface 23 b opposite to the optical surface23 a.

On the optical surface 23 b of the optical waveguide 23, there areprovided the first reflection volume hologram grating 24 in a positionwhere it is opposite to the light-incident port 23 a 1 at the opticalsurface 23 a and the second reflection volume hologram grating 25 in aposition where it is opposite to the light-outgoing port 23 a 2 at theoptical surface 23 a.

Also, the optical waveguide 23 has a transparent substrate 26 providedat the side thereof where the first and second reflection volumehologram gratings 24 and 25 are provided. Between the optical surface 23b of the optical waveguide 23 and the transparent substrate 26, therewill be laid an airspace Air in a place where the first and secondreflection volume hologram gratings 24 and 25 are not provided.

Because of the transparent substrate 26 provided as above, it ispossible to protect the optical surface 23 b being a total-reflectingsurface and the first and second reflection volume hologram gratings 24and 25.

FIGS. 12 and 13 show the first and second reflection volume hologramgratings 24 and 25 each having interference fringes recorded thereon.

As shown in FIGS. 12 and 13, each of the reflection volume hologramgratings 24 and 25 has recorded thereon a combination of three types ofinterference fringes that diffract and reflect mainly red light, greenlight and blue light, that is, red-light interference fringe 24R,green-light interference fringe 24G and blue-light interference fringe24B. The three types of interference fringes are recorded so thatgrating pitches on the hologram surfaces 24S and 25S will be equal foreach of the types of interference fringes and different from one type ofinterference fringe to another.

Note that the reflection volume hologram gratings 24 and 25 may have acombination of three types of interference fringes recorded on onehologram layer as shown in FIGS. 12 and 13 but each of the types ofinterference fringes, that is, each of a red-color interference fringe24R, green-light interference fringe 24G and blue-color interferencefringe 24B, may be recorded on one hologram layer and the three hologramlayers each having an interference fringe recorded thereon may bestacked together.

As shown in FIG. 12, the reflection volume hologram grating 24 has aplurality of interference fringes 24R, 24G and 24B recorded thereon atthe same slant with the same pitch independently of the locations of theinterference fringes. Similarly, the reflection volume hologram grating25 shown in FIG. 13 has a plurality of interference fringes 25R, 25G and25B at the same slant angle with the same pitch. Therefore, the firstand second reflection volume hologram gratings 24 and 25 have theirinterference fringes disposed on the optical surface 23 b of the opticalwaveguide 23 to be symmetrical with respect to a plane perpendicular tothe optical surface 23 b.

Parallel pencil groups incident upon the light-incident port 23 a 1 ofthe optical waveguide 23 are incident upon the above-mentioned firstreflection volume hologram grating 24 and diffracted and reflected theyare at a nearly same angle. The parallel pencil groups thus diffractedand reflected will travel while being repeatedly totally reflectedbetween the optical surfaces 23 a and 23 b of the optical waveguide 23and will be incident upon the above-mentioned second reflection volumehologram grating 25.

The length, and thickness between the optical surfaces 23 a and 23 b, ofthe optical waveguide 23 are set to provide such a thickness and to sucha sufficient length that parallel light beans traveling inside theoptical waveguide 23 while being totally reflected are totally reflecteddifferent numbers of times correspondingly to their angles of fielduntil they arrive at the second reflection volume hologram grating 25.

More specifically, ones of the parallel pencils in group incident uponthe optical waveguide 23 while being slanted toward the secondreflection volume hologram grating 25, that is, parallel pencilsincident at a larger angle, are reflected a smaller number of times thanparallel pencils incident upon the optical waveguide 23 while not beingslanted toward the second reflection volume hologram grating 25, thatis, parallel pencils incident at a smaller angle for the reason that theparallel pencil groups incident upon the optical waveguide 23 will bedifferent in angle of field from each other. Namely, since the parallelpencils are incident upon the first reflection volume hologram grating24 at different angles and thus projected at different angles ofdiffraction, they are totally reflected at different angles. Thus, whenthe optical waveguide 23 is designed slim having a sufficient length,the parallel pencils will be reflected different numbers of times,respectively.

Parallel pencil groups different in angle of field from each other andincident upon the second reflection volume hologram grating 25 arediffracted and reflected to depart from the condition of totalreflection, projected at the light-outgoing port 23 a 2 of the opticalwaveguide 23, and incident upon the pupil 16 of the viewer.

As above, the second reflection volume hologram grating 25 is providedon the optical surface 23 b of the optical waveguide 23 so that theinterference fringes recorded thereon take the same shape as that of theinterference fringes on the first reflecting volume hologram grating 24,rotated 180 deg. on the hologram surface. Therefore, since the parallelpencil groups to be reflected by the second reflection volume hologramgrating 25 will be reflected at an angle equal to the angle of incidenceupon the first reflecting volume hologram grating 24, a display imagewill be displayed on the pupil 16 with a high resolution withoutblurring.

Including the first and second reflection volume hologram gratings 24and 25 that do not act as any lens, the virtual image display device 20can eliminate monochromatic eccentric aberration and diffractionchromatic aberration.

Note that although the first and second reflection volume hologramgratings 24 and 25 are disposed for their hologram surfaces 24S and 25Sto be parallel to the optical surface 23 b of the optical waveguide 23,the present invention is not limited to this geometry but they may bedisposed for their hologram surfaces 24S and 25S to be at apredetermined angle in relation to the optical surface 23 b.

Third Embodiment

FIG. 14 shows a virtual image display device as a third embodiment ofthe present invention. The virtual image display device is generallyindicated with a reference numeral 40. The virtual image display device40 as the third embodiment of the present invention displays a virtualimage of a color image similarly to the virtual image display device 20as the aforementioned second embodiment. It should be noted that in FIG.14, only light rays directed at the central angle of field areillustrated mainly for easy viewing of the drawing.

The virtual image display device 40 includes an illumination opticalsystem 30 also used in the second embodiment, and a virtual imageoptical system to guide incident illumination light from theillumination optical system 30 to the pupil 16 of the viewer.

The virtual image optical system includes a collimating optical system22, reflecting mirror 45 to reflect a parallel pencil coming from thecollimating optical system 22, MEMS (Micro Mechanical System) mirror 41to make spatial modulation of the parallel pencil reflected by thereflecting mirror 45, optical waveguide 43 upon which the illuminationlight having been subjected spatial modulation by the MEMS mirror 41 anda first reflection volume hologram grating 44 and second reflectionvolume hologram grating 45 provided on the optical waveguide 43. Itshould be noted that the illumination optical system 30 and collimatingoptical system 22 having already been described will not be explainedany more.

The MEMS mirror 41 functions as a scanning optical system to form theparallel pencil into parallel pencil groups traveling in differentdirections by scanning the parallel pencil horizontally and vertically.

White light corning from the illumination optical system 30 goes asillumination light to the virtual image optical system to thecollimating optical system 22 which will form the light into a parallelpencil. The parallel pencil is projected to the reflecting mirror 45.

The reflecting mirror 45 is fixedly provided and projects the parallelpencil coming from the collimating optical system 22 to the MEMS mirror41.

The MEMS mirror 41 is a functional element prepared with thesemiconductor manufacturing technology. It works as a spatial opticalmodulator to make spatial modulation of an incident parallel pencil. TheMEMS mirror 41 is freely movable in a two-dimensional direction. Itforms an image through spatial modulation of an incident parallel pencilby reflecting the incident pencil so as to scan it in a directionperpendicular to the plane of the drawing and in a direction parallel tothe plane of the drawing. The MEMS mirror 41 operates under the controlof a microcomputer (not shown) correspondingly to an image to bedisplayed.

The parallel pencil coming from the MEMS mirror 41 is reflected asparallel pencil groups different in traveling direction from each othertoward the optical waveguide 43 while being scanned as they are in adirection perpendicular to the plane of the drawing and in a directionparallel to the drawing plane.

The optical waveguide 43 is a slim, parallel, flat optical waveguideincluding mainly an optical surface 43 b having provided at one endthereof a light-incident port 43 b 1 upon which there are incidentparallel pencil groups reflected by the MEMS mirror 41, and an opticalsurface 43 a opposite to the optical surface 43 b and having alight-outgoing port 43 a 1 provided at the end thereof opposite to thelight-incident port 43 b 1 of the optical surface 43 b.

The first reflection volume hologram grating 44 is provided on theoptical surface 43 a of the optical waveguide 43 in a position where itis opposite to the light-incident port 43 b 1 at the optical surface 43b, and the second reflection volume hologram grating 45 is provided onthe optical surface 43 b in a position where it is opposite to thelight-outgoing port 43 a 1 at the optical surface 43 a.

FIGS. 15 and 16 show the first and second reflection volume hologramgratings 44 and 45 each having interference fringes recorded thereon.These first and second reflection volume hologram gratings 44 and 45 arequite the same in configuration as the first and second reflectionvolume hologram gratings 24 and 25 having already been described withreference to FIGS. 12 and 13 except that the first and second reflectionvolume hologram gratings 44 and 45 are provided in the positions wherethe second and first reflection volume hologram grating 25 and 24 areprovided, respectively.

As shown in FIGS. 15 and 16, each of the reflection volume hologramgratings 44 and 45 has recorded thereon a combination of three types ofinterference fringes that diffract and reflect mainly red light, greenlight and blue light, that is, red-light interference fringe 44R,green-light interference fringe 44G and blue-light interference fringe44B. The three types of interference fringes are recorded so thatgrating pitches on the hologram surfaces will be equal for each of thetypes of interference fringes and different from one type of theinterference fringe to another.

Note that the reflection volume hologram gratings 44 and 45 may have acombination of three types of interference fringes recorded on onehologram layer as shown in FIGS. 15 and 16 but each of the types ofinterference fringes, that is, each of a red-color interference fringe44R, green-light interference fringe 44G and blue-color interferencefringe 44B, may be recorded on one hologram layer and the three hologramlayers each having an interference fringe recorded thereon may bestacked together.

As shown in FIG. 15, the first reflection volume hologram grating 44 hasa plurality of interference fringes 44R, 44G and 44B recorded thereon atthe same slant with the same pitch independently of the locations of theinterference fringes. Similarly, the second reflection volume hologramgrating 45 shown in FIG. 16 has a plurality of interference fringes 45R,45G and 45B at the same slant angle with the same pitch.

Parallel pencil groups incident from the light-incident port 43 b 1 ofthe optical waveguide 43 are incident upon the above-mentioned firstreflection volume hologram grating 44 and diffracted and reflected asthey are at a nearly same angle. The parallel pencil groups thusdiffracted and reflected will travel while being repeatedly totallyreflected between the optical surfaces 43 a and 43 b of the opticalwaveguide 43 and will be incident upon the above-mentioned secondreflection volume hologram grating 45.

The length, and thickness between the optical surfaces 43 a and 43 b, ofthe optical waveguide 43 are set to provide such a thickness and to sucha sufficient length that parallel light beans traveling inside theoptical waveguide 43 while being totally reflected are totally reflecteddifferent numbers of times correspondingly to their angles of fielduntil they arrive at the second reflection volume hologram grating 45.

More specifically, ones of the parallel pencil groups incident upon theoptical waveguide 43 while being slanted toward the second reflectionvolume hologram grating 45, that is, parallel pencils incident at alarger angle, are reflected a smaller number of times than parallelpencils incident upon the optical waveguide 43 while not being slantedtoward the second reflection volume hologram grating 45, that is,parallel pencils incident at a smaller angle for the reason that theparallel pencil groups incident upon the optical waveguide 43 will bedifferent in angle of field from each other. Namely, since the parallelpencils are incident upon the first reflection volume hologram grating44 at different angles and thus projected at different angles ofdiffraction, they are totally reflected at different angles. Thus, whenthe optical waveguide 43 is designed slim having a sufficient length,the parallel pencils will be reflected different numbers of times,respectively.

Parallel pencil groups different in angle of field from each other andincident upon the second reflection volume hologram grating 45 arediffracted and reflected to depart from the condition of totalreflection, projected at the light-outgoing port 43 a 1 of the opticalwaveguide 43, and incident upon the pupil 16 of the viewer.

As above, the second reflection volume hologram grating 45 is providedon the optical surface 43 b of the optical waveguide 43 so that theinterference fringes recorded thereon take the same shape as that of theinterference fringes on the first reflecting volume hologram grating 44,rotated 360 deg. on the hologram surface. Therefore, since the parallelpencil groups to be reflected by the second reflection volume hologramgrating 45 will be reflected at an angle equal to the angle of incidenceupon the first reflecting volume hologram grating 44, a display imagewill be displayed on the pupil 16 with a high resolution withoutblurring.

Including the first and second reflection volume hologram gratings 44and 45 that do not act as any lens, the virtual image display device 40can eliminate monochromatic eccentric aberration and diffractionchromatic aberration.

Note that although the first and second reflection volume hologramgratings 44 and 45 are disposed for their hologram surfaces 44S and 45Sto be parallel to the optical surfaces 43 a and 43 b, respectively, ofthe optical waveguide 43, the present invention is not limited to thisgeometry but they may be disposed for their hologram surfaces 44S and45S to be at a predetermined angle in relation to the optical surfaces43 a and 43 b, respectively.

Fourth Embodiment

FIG. 17 shows a virtual image display device as a fourth embodiment ofthe present invention. The virtual image display device is generallyindicated with a reference numeral 60. The virtual image display device60 as the fourth embodiment of the present invention displays a virtualimage of a color image. It should be noted that in FIG. 17, only lightrays directed at the central angle of field are illustrated mainly foreasy viewing of the drawing.

The virtual image display device 60 includes an illumination opticalsystem 70, spatial optical modulator 61 to make spatial modulation ofillumination light from the illumination optical system 70, and avirtual image optical system to guide the incident illumination lightspatial-modulated by the spatial optical modulator 61 to the pupil 16 ofthe viewer.

The illumination optical system 70 includes a laser light source 71R toemit red light, laser light source 71G to emit green light, laser lightsource 71B to emit blue light, color synthesizing prism 72, couplingoptical system 73, speckle reducing means 74, optical fiber 75 and acondenser lens 76.

Red, green and blue light emitted from the laser light sources 71R, 71Gand 71B, respectively, are mixed by the color synthesizing prism 32 thatis a cross prism to provide white light, and the white light is led bythe coupling optical system 73 to the optical fiber 75 via the specklereducing means 74. The white light transmitted through the optical fiber75 and projected from the latter illuminates the spatial opticalmodulator 61 via the condenser lens 76.

The spatial optical modulator 61 is for example a transmission liquidcrystal display to make spatial modulation of the incident illuminationlight per pixel. The illumination light thus spatial-modulated isincident upon the virtual image optical system.

The virtual image optical system includes a collimating optical system62, optical waveguide 63, and first and second reflection volumehologram gratings 64 and 65 provided on the optical waveguide 63.

The collimating optical system 62 forms incident illumination lightspatial-modulated by the spatial optical modulator 61 into parallelpencil groups different in angle of field from each other. The parallelpencil groups coming from the collimating optical system 62 is incidentupon the optical waveguide 63.

The optical waveguide 63 is a slim, parallel, flat optical waveguideincluding mainly an optical surface 63 a having provided at one endthereof a light-incident port 63 a 1 upon which there are incidentparallel pencil groups coming from the collimating optical system 62,and at the other end a light-outgoing port 63 a 2 from which the lightis projected, and an optical surface 63 b opposite to the opticalsurface 63 a.

On the optical surface 63 b of the optical waveguide 63, there areprovided the first reflection volume hologram grating 64 in a positionwhere it is opposite to the light-incident port 63 a 1 at the opticalsurface 63 a and the second reflection volume hologram grating 65 in aposition where it is opposite to the light-outgoing port 63 a 2 at theoptical surface 63 a.

FIGS. 18 and 19 show the first and second reflection volume hologramgratings 64 and 65 each having interference fringes recorded thereon.

As shown in FIGS. 18 and 19, the first and second reflection volumehologram gratings 64 and 65 are formed each from three hologram layers64A, 64B and 64C stacked together and three hologram layers 65A, 65B and65C stacked together, respectively. Each of the hologram layers of eachreflection volume hologram grating has recorded thereon interferencefringes to diffract and reflect mainly red, green and blue light. Forexample, the hologram layer 64A of the first reflection volume hologramgrating 64 has recorded thereon an interference fringe to diffract andreflect mainly red light, the hologram layer 64B has recorded thereon aninterference fringe to diffract and reflect mainly green light, and thehologram layer 64C has recorded thereon an interference fringe todiffract and reflect mainly blue light. This is also true for the secondreflection volume hologram grating 65.

Also, the inference fringe recorded on each hologram layer is formedfrom a combination of three types of interference fringes different inslant angle from each other and laid with the same pitch on the hologramsurface for a larger diffraction acceptance angle in relation to aparallel pencil of a waveband to be diffracted and reflected by eachhologram layer as in the interference fringe recorded on the first andsecond reflection volume hologram gratings 14 and 15 in the firstembodiment.

Also, the first and second reflection volume hologram gratings 64 and 65may be constructed as below. This will be explained concerning thesecond reflection volume hologram grating 65 with reference to FIG. 20.It should be noted that although the first reflection volume hologramgrating 64 will not be explained at all, it is quite the same inconstruction as the second reflection volume hologram grating 65.

As shown in FIG. 20, the second reflection volume hologram grating 65 isformed from a stack of three hologram layers 65D, 65E and 65F. Each ofthe hologram layers forming together the second reflection volumehologram grating 65 has recorded thereon a combination of three types ofinterference fringes for diffraction and reflection of rays of lightdifferent in waveband from each other for a wider range of diffractionacceptance wavelength. The three types of interference fringes arerecorded for the grating pitch on the hologram surface to be equal foreach type of the interference fringe and different from one type ofinterference fringe to another. Namely, each hologram layer of thesecond reflection volume hologram grating 65 has recorded thereonsimilar interference fringes to those on the first and second reflectionvolume hologram gratings 24 and 25 used in the second embodiment.

Also, the interference fringes recorded on the hologram layers 65D, 65Eand 65F have slant angles θd, θe and θf. The slant angles are quiteidentical to each other in the same hologram layer, but they aredifferent from one hologram layer to another for a larger diffractionacceptance angle.

Parallel pencil groups incident from the light-incident port 63 a 1 ofthe optical waveguide 63 are incident upon the above-mentioned firstreflection volume hologram grating 64 and diffracted and reflected. Theparallel pencil groups thus diffracted and reflected will travel whilebeing repeatedly totally reflected between the optical surfaces 63 a and63 b of the optical waveguide 63 and will be incident upon theabove-mentioned second reflection volume hologram grating 65.

The length, and thickness between the optical surfaces 63 a and 63 b, ofthe optical waveguide 63 are set to provide such a thickness and to sucha sufficient length that parallel light beans traveling inside theoptical waveguide 63 while being totally reflected are totally reflecteddifferent numbers of times correspondingly to their angles of fielduntil they arrive at the second reflection volume hologram grating 65.

More specifically, ones of the parallel pencil groups incident upon theoptical waveguide 63 while being slanted toward the second reflectionvolume hologram grating 65, that is, parallel pencils incident at alarger angle, are reflected a smaller number of times than parallelpencils incident upon the optical waveguide 63 while not being slantedtoward the second reflection volume hologram grating 65, that is,parallel pencils incident at a smaller angle for the reason that theparallel pencils incident upon the optical waveguide 63 will bedifferent in angle of field from each other. Namely, since the parallelpencils are incident upon the first reflection volume hologram grating64 at different angles and thus projected at different angles ofdiffraction, they are totally reflected at different angles. Thus, whenthe optical waveguide 63 is designed slim having a sufficient length,the parallel pencils will be reflected considerably different numbers oftimes, respectively.

Parallel pencil groups different in angle of field from each other andincident upon the second reflection volume hologram grating 65 isdiffracted and reflected to depart from the condition of totalreflection, projected at the light-outgoing port 63 a 2 of the opticalwaveguide 63, and incident upon the pupil 16 of the viewer.

As above, the second reflection volume hologram grating 65 is providedon the optical surface 63 b of the optical waveguide 63 so that theinterference fringes recorded thereon take the same shape as that of theinterference fringes on the first reflecting volume hologram grating 64,rotated 180 deg. on the hologram surface. Therefore, since the parallelpencil groups to be reflected by the second reflection volume hologramgrating 65 will be reflected at an angle equal to the angle of incidenceupon the first reflecting volume hologram grating 64, a display imagewill be displayed on the pupil 16 with a high resolution withoutblurring.

Including the first and second reflection volume hologram gratings 64and 65 that do no act as any lens, the virtual image display device 60can eliminate monochromatic eccentric aberration and diffractionchromatic aberration.

Note that although the first and second reflection volume hologramgratings 64 and 65 are disposed for their hologram surfaces 64S and 65Sto be parallel to the optical surface 63 b of the optical waveguide 63,the present invention is not limited to this geometry but they may bedisposed for their hologram surfaces 64S and 65S to be at apredetermined angle in relation to the optical surface 63 b.

Fifth Embodiment

FIG. 21 shows a virtual image display device as a fifth embodiment ofthe present invention. The virtual image display device is generallyindicated with a reference numeral 80. The virtual image display device80 includes an image display element 81 to display an image, and avirtual image optical system to guide incident display light from theimage display element 81 to a pupil 16 of the viewer.

The image display element 81 is for example an organic EL (ElectroLuminescence) display, inorganic EL display, liquid crystal display(LCD) or the like.

The virtual image optical system includes a collimating optical system82 and an optical waveguide 83 which incorporates a hologram layer 84.

The collimating optical system 82 is to receive a pencil from each pixelof the image display element 81 and form the pencils into parallelpencil groups different in angle of field from each other. The parallelpencil groups coming from the collimating optical system 82 anddifferent in angle of field from each other are incident upon theoptical waveguide 83.

The optical waveguide 83 is of a structure in which the hologram layer84 is laid between transparent substrates 83A and 83B. The opticalwaveguide 83 is a slim, parallel, flat optical waveguide includingmainly an optical surface 83 a having provided at one end thereof alight-incident port 83 a 1 upon which there are incident parallel pencilgroups projected from the collimating optical system 82 and different inangle of field from each other and at the other end a light-outgoingport 83 a 2 from which the light is projected, and an optical surface 83b opposite to the optical surface 83 a.

Protective sheets 85 and 86 are provided on the optical surfaces 83 aand 83 b, respectively, of the optical waveguide 83 to protect theoptical surfaces 83 a and 83 b, respectively. The protective sheet 86provided on the optical surface 83 b has provided thereon in the sameposition as the light-incident port 83 a 1 of the optical waveguide 83 alight shield 87 that prevents the efficiency of light utilization frombeing reduced by leakage of an image displayed on the image displayelement 81 and magnified by the collimating optical system 82 to outsidethe optical waveguide 83.

The hologram layer 84 has a first reflection volume hologram grating 84a in a position corresponding to the light-incident port 83 a 1 and asecond reflection volume hologram grating 84 c in a positioncorresponding to the light-outgoing port 83 a 2. The rest of thehologram layer 84 is an area 84 b having no interference fringe recordedtherein.

Each of the first and second reflection volume hologram gratings 84 aand 84 c has interference fringes with an identical pitch of thehologram surface. Also, the second reflection volume hologram grating 84c is designed to be different in diffraction efficiency from one portionthereof to another. The second reflection volume hologram grating 84 cis lower in diffraction efficiency in a position near the light-incidentport 83 a 1 and higher in diffraction efficiency in a position distantfrom the light-incident port 83 a 1 so that the light can be diffractedand reflected a plurality of times.

Parallel pencil groups incident upon the light-incident port 83 a 1 ofthe optical waveguide 83 and different in angle of field from each otherare incident upon the above-mentioned first reflection volume hologramgrating 84 a and diffracted and reflected as they are. The parallelpencil groups thus diffracted and reflected will travel while beingrepeatedly totally reflected between the optical surfaces 83 a and 83 bof the optical waveguide 83 and will be incident upon theabove-mentioned second reflection volume hologram grating 84 c.

The length, and thickness between the optical surfaces 83 a and 83 b, ofthe optical waveguide 83 are set to provide such an optical length thatparallel light beans different in angle of field from each other andtraveling inside the optical waveguide 83 are totally reflecteddifferent numbers of times correspondingly to their angles of fielduntil they arrive at the second reflection volume hologram grating 84 c.

More specifically, ones of the parallel pencil groups incident upon theoptical waveguide 83 while being slanted toward the second reflectionvolume hologram grating 84 c, that is, parallel pencils incident at alarger angle, are reflected a smaller number of times than parallelpencils incident upon the optical waveguide 83 while not being slantedtoward the second reflection volume hologram grating 84 c, that is,parallel pencils incident at a smaller angle for the reason that theparallel pencils incident upon the optical waveguide 83 will bedifferent in angle of field from each other. Namely, since the parallelpencils are incident upon the first reflection volume hologram grating84 a at different angles and thus projected at different angles ofdiffraction, they are totally reflected at different angles. Thus, whenthe optical waveguide 83 is designed slim having a sufficient length,the parallel pencils will be reflected different numbers of times,respectively.

Parallel pencil groups different in angle of field from each other andincident upon the second reflection volume hologram grating 84 c arediffracted and reflected to depart from the condition of totalreflection, projected at the light-outgoing port 83 a 2 of the opticalwaveguide 83, and incident upon the pupil 16 of the viewer.

In case the second reflection volume hologram grating 84 c is designedto be different in diffraction efficiency from one portion thereon toanother as in this embodiment, the pupil diameter, that is, the viewer'svirtual image viewing range, can be increased.

More specifically, on the assumption that the diffraction efficiency ofthe second reflection volume hologram grating 84 c is 40% in a position84 c 1 near the light-incident port 83 a 1 and 70% in a position 84 c 2distant from the light-outgoing port 83 a 2, for example, a first groupof parallel pencils incident upon the second reflection volume hologramgrating 84 c will have 40% thereof diffracted and reflected in theposition 84 c 1 and 60% allowed to pass by. The parallel pencil groupsallowed to pass by will be totally reflected inside the opticalwaveguide 83 and incident upon the second reflection volume hologramgrating 84 c in the position 84 c 2.

Since the diffraction efficiency in the position 84 c 2 is 70%, 60% ofthe first group of parallel pencils incident upon the second reflectionvolume hologram grating 84 c is allowed to pass by. Thus, 42%(=0.6×0.7=0.42) of the parallel pencil groups will be diffracted andreflected in the position 84 c 2. By changing the diffraction efficiencyappropriately from one position to another on the second reflectionvolume hologram grating 84 c as above, it is possible to keep thebalance in amount of the light coming from the light-outgoing port 83 a2. Therefore, by increasing the area of the second reflection volumehologram grating 84 c where the interference fringe is to be recorded,the range of virtual image viewing range can easily be increased.

Also, the virtual image display device 80 with the first and secondreflection volume hologram gratings 84 a and 84 c that do not act as anylens can eliminate the monochromatic eccentric aberration anddiffraction chromatic aberration.

Sixth Embodiment

FIG. 22 shows a virtual image display device as a sixth embodiment ofthe present invention. The virtual image display device is generallyindicated with a reference numeral 90. The virtual image display device90 includes an image display element 91 to display an image, and avirtual image optical system to guide incident display light from theimage display element 91 to a pupil 16 of the viewer.

The image display element 91 is for example an organic EL (ElectroLuminescence) display, inorganic EL display, liquid crystal display(LCD) or the like.

The virtual image optical system includes a collimating optical system92, optical waveguide 93, and a first reflection volume hologram grating94 and second reflection volume hologram grating 95 provided on theoptical waveguide 93.

The collimating optical system 92 receives an incident pencil from eachpixel of the image display element 91 and forms the pencils of rays intoparallel pencil groups different in angle of field from each other. Theparallel pencil groups projected from the collimating optical system 92and different in angle of field from each other is incident upon theoptical waveguide 93.

The optical waveguide 93 is a slim, parallel, flat optical waveguideincluding mainly an optical surface 93 a having provided at one endthereof a light-incident port 93 a 1 upon which there are incidentparallel pencil groups projected from the collimating optical system 92and different in angle of field from each other and at the other end alight-outgoing port 93 a 2 from which the light is projected, and anoptical surface 93 b opposite to the optical surface 93 a.

On the optical surface 93 b of the optical waveguide 93, there areprovided the first reflection volume hologram grating 94 in a positionwhere it is opposite to the light-incident port 93 a 1 at the opticalsurface 93 a and the second reflection volume hologram grating 95 in aposition where it is opposite to the light-outgoing port 93 a 2 at theoptical surface 93 a.

The first and second reflection volume hologram gratings 94 and 95 willbe described in detail later.

Parallel pencil groups coming from the light-incident port 63 a 1 of theoptical waveguide 93 and different in angle of field from each other areincident upon the above-mentioned first reflection volume hologramgrating 94 and diffracted and reflected as they are. The parallel pencilgroups thus diffracted and reflected will travel while being repeatedlytotally reflected between the optical surfaces 93 a and 93 b of theoptical waveguide 93 and will be incident upon the above-mentionedsecond reflection volume hologram grating 95.

The length, and thickness between the optical surfaces 93 a and 93 b, ofthe optical waveguide 93 are set to provide such a thickness and to sucha sufficient length that parallel light beans different in angle offield from each other and traveling inside the optical waveguide 93while being totally reflected are totally reflected different numbers oftimes correspondingly to their angles of field until they arrive at thesecond reflection volume hologram grating 95.

More specifically, ones of the parallel pencil groups incident upon theoptical waveguide 93 while being slanted toward the second reflectionvolume hologram grating 95, that is, parallel pencils incident at alarger angle, are reflected a smaller number of times than parallelpencils incident upon the optical waveguide 93 while not being slantedtoward the second reflection volume hologram grating 95, that is,parallel pencils incident at a smaller angle for the reason that theparallel pencil groups incident upon the optical waveguide 93 will bedifferent in angle of field from each other. Namely, since the parallelpencils are incident upon the first reflection volume hologram grating94 at different angles and thus projected at different angles ofdiffraction, they are totally reflected at different angles. Thus, whenthe optical waveguide 93 is designed slim having a sufficient length,the parallel pencils will be reflected different numbers of times,respectively.

Parallel pencil groups different in angle of field from each other andincident upon the second reflection volume hologram grating 95 arediffracted and reflected to depart from the condition of totalreflection, projected at the light-outgoing port 93 a 2 of the opticalwaveguide 93, and incident upon the pupil 16 of the viewer.

Next, the first and second reflection volume hologram gratings 94 and 95will be described.

The reflection volume hologram grating 94 is quite the same inconstruction (not illustrated) as the first reflection volume hologramgrating 64 in the fourth embodiment having already been described withFIG. 18. Therefore, the first reflection volume hologram grating 94 isformed from a stack of three hologram layers different in pitch ofinterference fringe to diffract and reflect red, green and blue light.Each of the hologram layers is formed from a combination of three typesof interference fringes different in slant angle from each other for alarger angle of field and laid with the same pitch on the hologramsurface.

Thus, the first reflection volume hologram grating 94 can diffract andreflect parallel pencils projected from the image display element 91 andcollimated by the collimating optical system 92 to have a horizontalangle of field±about 10 deg. so as to meet the condition of totalreflection for the optical waveguide 93.

The parallel pencil groups diffracted and reflected by the firstreflection volume hologram grating 94 will be guided inside the opticalwaveguide 93 while being totally reflected at different angles,respectively. As a result, the parallel pencils will be incident atdifferent angles upon the second reflection volume hologram grating 95.

FIG. 23 shows parallel pencil groups diffracted and reflected by thefirst reflection volume hologram grating 94, totally reflected insidethe optical waveguide 93 and incident upon the second reflection volumehologram grating 95. The parallel pencils are incident upon the secondreflection volume hologram grating 95 at different angles depending uponthe position of incidence as shown in FIG. 23.

More specifically, upon a position of the second reflection volumehologram grating 95, near the first reflection volume hologram grating94, there are incident both a parallel pencil being a parallel pencil LLguided by internal total reflection at a large angle and having such anangle of field that the beam has been internally total-reflected a smallnumber of times and a parallel pencil being a parallel pencil LS guidedby internal total reflection at a small angle and having such an angleof field that the beam has been internally total-reflected a largenumber of times.

Note that the parallel pencil indicated with a dashed line in FIG. 23 isa parallel pencil LM having been guided by internal total reflection atan angle that is intermediate between the angles of total reflection ofthe parallel pencil LL having been guided by the internal totalreflection at the large angle and parallel pencil LS having been guidedby the internal total reflection at the small angle.

On the other hand, upon a position of the second reflection volumehologram grating 95, distant from the first reflection volume hologramgrating 94, there are incident mainly the parallel pencil LS having beenguided by internal total reflection at a small angle.

That is, the parallel pencil incident upon each position on the secondreflection volume hologram grating 95 will have the incident anglehereof determined to some extent. For example, it is assumed here thatthe second reflection volume hologram grating 95 has recorded thereonsuch an interference fringe as will evenly diffract and reflect aparallel pencil incident at an angle having some range in any positionlike the first reflection volume hologram grating 94. This is effectivefor increasing the pupil diameter, but in case the pupil has a fixeddiameter, the light amount incident upon the pupil 16 will be reducedand the display image provided to the viewer will be very dark.

On this account, the second reflection volume hologram grating 95 isdesigned, based on the fact that the incident angle of a parallel pencilvaries depending upon the incident position, to have recorded thereonsuch an interference fringe that a parallel pencil incident at an anglecorresponding to its incident position will be diffracted with themaximum efficiency.

For example, the second reflection volume hologram grating 95 is formedfrom a stack of hologram layers 95A, 95B and 95C each having aninterference fringe as shown in FIG. 24. The three hologram layers 95A,95B and 95C have recorded thereon interference fringes different ingrating pitch from each other to diffract and reflect mainly any of red,green and blue light, respectively.

Next, the interference fringe recorded on the hologram layer 95C of thesecond reflection volume hologram grating 95 shown in FIG. 24 will beexplained with reference to FIG. 25. It should be noted that theinterference fringes recorded on the hologram layers 95A and 95B willnot be explained because they are similar to that recorded on thehologram layer 95C except that they are recorded with grating pitchesdifferent from that of the interference fringe on the hologram layer95C. Also it should be noted that in FIG. 25, the side of the hologramlayer 95C near the first reflection volume hologram grating 94 whenprovided on the optical waveguide 93 is taken as “R” side and the sideopposite to this “R” side is taken as “L” side.

At the R side of the hologram layer 95C, an interference fringe 95Rwhose slant angle θR is small is recorded to a region R for a higherefficiency of diffracting a parallel pencil incident at a large angle.Also, at the L side, an interference fringe 95L whose slant angle θL islarge is recorded to a region L for a higher efficiency of diffracting aparallel pencil incident at a small angle. An interference fringe 95Mwhose slant angle θM is intermediate between the slant angles θR and θLis recorded in a region M between the R and L sides.

The interference fringes 95R, 95L and 95M are different in slant anglefrom each other as above. However, they are laid with the same gratingpitch on a hologram surface 95CS. Unless all the interference fringesare laid with the same grating pitch, parallel pencils incident with thesame wavelength and at the same angle will be diffracted and reflectedat different angles of diffraction. Such parallel pencils arriving atthe pupil 16 of the viewer will form in a low-resolution, out-of-focusimage.

The interference fringe recorded on each of the hologram layers 95A and95B is a combination of three types of interference fringes different inslant angle from each other as in the hologram layer 95C except that thegrating pitch is altered for diffraction and reflection of a parallelpencil of a waveband different from that mainly for diffraction andreflection by the hologram layer 95C.

The hologram layer 95C shown in FIG. 25 has recorded thereon acombination of three types of interference fringes. It should be notedhowever that a stack of holograms having interference fringes 95R, 95Land 95M recorded thereon respectively as shown in FIG. 26 provides quitethe same effect.

In the hologram layer 95C shown in FIG. 26, the hologram layers 95CR,95CL and 95CM have the interference fringes 95R, 95L and 95M recordedthereon respectively, and the hologram layer 95CM is stacked in theintermediate position between the hologram layers 95CR and 95CL laidhorizontally.

As mentioned above, by altering the slant angle of the interferencefringe recorded on each of the regions R, L and M of the hologram layerincluded in the second reflection volume hologram grating 95, thediffraction can be made with a maximum efficiency correspondingly to theincident angle of an incident parallel pencil. On this account, a slantangle of an interference fringe for the maximum diffraction efficiencywill be explained below taking a reflection volume hologram grating 96shown in FIG. 27 as an example.

For the convenience of explanation of the slant angle, it is assumedthat the reflection volume hologram grating 96 shown in FIG. 27 isprovided in place of the second reflection volume hologram grating 95 inthe virtual image display device 90 shown in FIG. 25 and the pencil istraced starting at the pupil 16 of the viewer on the basis of the factthat the reflection volume hologram grating is reversible in property.Namely, the explanation will be given on the assumption that displaylight projected from a virtually provided image display element iscollimated by the collimating optical system into a parallel pencilhaving a horizontal angle of field±about 10 deg. and incident upon thereflection volume hologram grating 96 shown in FIG. 27. In this case,the incident light upon the reflection volume hologram grating 96corresponds to the light diffracted and reflected by the secondreflection volume hologram grating 95 and the light diffracted andreflected by the reflection volume hologram grating 96 corresponds tothe incident light upon the second reflection volume hologram grating95.

For diffraction and reflection of all incident parallel light-beams tomeet a condition of total reflection inside the optical waveguide 93 incase the parallel light-beams of a horizontal angle of field±about 10deg. are incident upon the reflection volume hologram grating 96, theangle of diffraction and reflection has to be 55 to 60 deg. when thereare incident a parallel pencil Lp at the central angle of field isincident at an angle of 0 deg.

That is, when the angle of diffraction and reflection is other than 55to 60 deg. in case the parallel pencil Lp is incident at 0 deg., some ofthe parallel pencils incident at an angle other than 0 deg within arange of ±10 deg. will be diffracted and reflected at an angle notmeeting the condition of total reflection inside the optical waveguide93.

It is assumed here that in the hologram region 96M of the reflectionvolume hologram grating 96 shown in FIG. 27, there is recorded aninterference fringe that diffracts and reflects, at angle θk of 55 to 60deg., the parallel pencil Lp incident at an angle of 0 deg. upon thehologram region 96M. It should be noted that the angle θk of diffractionand reflection is represented by an angle of projection diffraction as apractice of coordinate definition as shown in FIG. 27. The angle θs is120 to 125 deg.

The angle θr of incidence upon the hologram region 96M where the aboveinterference fringe is recorded and angle θs of projection diffractioncan be given by the following equation (1):sin θs=sin θr+λ/Λp  (1)where λ is the wavelength of the incident parallel pencil and Λp is thegrating pitch of the interference fringe on the hologram surface.

Also, in case the grating pitch Λp of the recorded interference fringemeets the equation (1), the slant angle φ0 of the interference fringefor a maximum diffraction efficiency when a parallel pencil is incidentat the angle θr and diffracted and reflected at the angle θs can begiven by the following equation (2) based on the Bragg condition:φ0=(θs+θr)/2  (2)

Since it is when the parallel pencil is specular-reflected by theinterference fringe that the parallel pencil incident at an angle θ andhaving been diffracted and reflected at the angle θs is diffracted witha maximum efficiency, the slant angle φ0 is also given by the equation(2).

Note here that the permissible range of incident angle for keeping themaximum diffraction efficiency of the grating of the reflection volumehologram grating is normally 0±3 deg. as shown in FIG. 6. Therefore, forany parallel pencil incident at an angle larger or smaller than above, anew interference fringe slanted at a different angle has to be recordedfor diffraction and reflection at a maximum efficiency.

At this time, the grating pitch of the new interference fringe to berecorded has to be the same as that of the existing interferencefringes. If the grating pitch of the interference fringes is altered,when parallel pencils of the same wavelength are incident at the sameangle, the angle of projection diffraction will vary at the respectiveinterference fringes, leading to a lower resolution.

Recording of interference fringes other than recorded in the hologramregion 96M to the reflection volume hologram grating 96 will beconsidered. More specifically, it is assumed that there is newlyrecorded such an interference fringe which will radiate and diffract atan angle θc′ when a parallel pencil having a wavelength 2 is incident atan angle θc within a range of ±10 deg. or so. The grating pitch of thenew interference fringe should be the same as that Λp of theinterference fringe pre-recorded in the hologram region 96M.

At this time, the angle θc′ of projection and diffraction is sin θc′=sinθc+λ/Λp: (λ/Λp=C) and can be given by the following equation (3):θc′=arc sin(sin θc+C)  (3)

At this time, the slant angle φc for the maximum diffraction efficiencyis φc=(θc′+θc)/2, and thus can be given by the following equation (4):φc={arc sin(sin θc+C)+θc}/2  (4)where C=λ/Λp.

FIG. 28 shows a plot of the variation, plotted using the above equation(4), of the slant angle φc of the interference fringe for maximumdiffraction efficiency when a parallel pencil is incident at an angle θcchanged within a range of about ±10 deg. The grating pitch Λp can becalculated by the equation (1). In FIG. 28, the dashed line A indicatesthe slant angle φc for the maximum efficiency of diffraction of parallelpencil incident at the angle θc when the grating pitch Λp is determinedwith an incident angle θr of 0 deg. and projection and diffraction angleθs of 125 deg. The solid line B in FIG. 28 indicates the slant angle φcfor the maximum efficiency of diffraction of parallel pencil incident atthe angle θc when the grating pitch Λp is determined with an incidentangle θr of 0 deg. and projection and diffraction angle θs of 120 deg.

As shown in FIG. 28, the slant angle c for the maximum diffractionefficiency is larger with a parallel pencil incident at a negative-goingangle of field and smaller with a parallel pencil incident at apositive-going angle of field.

As shown in FIG. 27 for example, when an interference fringe of asmaller slant angle θc than that of the interference fringes recorded inthe hologram region 96M is recorded with a grating pitch Λp in thehologram region 96R of the reflection volume hologram grating 96, theparallel pencil incident at a positive-going angle of field can bediffracted with a maximum efficiency.

Also, when an interference fringe of a larger slant angle θc than thatof the interference fringes recorded in the hologram region 96M isrecorded with a grating pitch Λp in the hologram region 96L of thereflection volume hologram grating 96, the parallel pencil incident at anegative-going angle of field can be diffracted with a maximumefficiency.

Therefore, the second reflection volume hologram grating 95 included inthe virtual image display device 90 shown in FIG. 24 can provide veryefficient diffraction and reflection with the slant angle of theinterference fringe near the first reflection volume hologram grating 94being reduced while the slant angle of the interference fringe distantfrom the first reflection volume hologram grating 94 is increased. Thus,the light amount of an image incident as a virtual image to the pupil ofa predetermined diameter can be increased considerably.

Provided with the first and second reflection volume hologram gratings94 and 95 that do not act as any lens, the virtual image display device90 can eliminate and reduce the monochromatic eccentric aberration anddiffraction chromatic aberration.

Note that although the first and second reflection volume hologramgratings 94 and 95 are disposed for their hologram surfaces to beparallel to the optical surface 93 b of the optical waveguide 93, thepresent invention is not limited to this geometry but they may bedisposed for their hologram surfaces to be at a predetermined angle inrelation to the optical surface 93 b.

Since the optical waveguide included in the virtual image opticaldevices having been described as the first to sixth embodiments of thepresent invention can be designed slimmer, so this virtual image displaydevice used as an HMD (Head Mounted Display) will be considerably lessuncomfortable to the viewer wearing the HMD.

Note that although the first to sixth embodiments of the presentinvention have been described and illustrated in the foregoing as oneshaving the optical waveguides 13, 23, 43, 63, 83 and 93, respectively,that are all a slim, parallel, flat one, the present invention is notlimited to any such slim optical waveguide but a optical waveguide,gently curved, can be equal in effect to the parallel, flat opticalwaveguides.

Also, in the aforementioned first to sixth embodiments of the presentinvention, the collimating optical system may be formed from acombination of a reflection optical element and optical lens, forexample, for a more compact and lightweight device. There will bedescribed the seventh to fifteenth embodiments of the present inventionadopting a collimator compact and of which the curvature of field isextremely small for an improved resolution of the image display deviceand a more compact and lightweight of the entire device.

Seventh Embodiment

FIG. 29 shows an image display device as a seventh embodiment of thepresent invention. The image display device is generally indicated witha reference numeral 100.

The image display device 100 includes an illumination light source 101,reflection spatial optical-modulating element 104 to reflectillumination light emitted from the illumination light source 101 andmake spatial modulation of the light, and a virtual image optical systemto receive the illumination light spatial-modulated by the reflectionspatial optical-modulating element 104 and guide it to the pupil 16 ofthe viewer.

The reflection spatial optical modulator 104 is for example a reflectionliquid crystal display or the like, and it makes spatial modulation ofincident illumination light per pixel. The spatial-modulatedillumination light is incident upon the virtual image optical system.

The virtual image optical system includes a collimating optical system,optical waveguide 120, and first and second reflection hologram elements123 and 124 provided on the optical waveguide 120.

The first and second reflection hologram elements 123 and 124 aredesigned similarly to the first and second reflection volume hologramgratings 14 and 15 shown in FIGS. 5, 6 and 7, for example.

Note that the first and second reflection hologram elements 123 and 124may be constructed similarly to the first ad second reflection volumehologram gratings 24 and 25 shown in FIGS. 8, 9 and 10. Also, they maybe constructed similarly to the first and second reflection volumehologram gratings 44 and 45 shown in FIGS. 11, 12 and 13. Moreover, theymay be constructed similarly to the first and second reflection volumehologram gratings 64 and 65 shown in FIGS. 14, 15 and 16. Also, they maybe constructed similarly to the first and second reflection volumehologram gratings 94, 95 and 96 shown in FIGS. 20, 21 and 24.

The collimating optical system includes an aspheric concave mirror 107as a reflection optical element to reflect light from the reflectiontype spatial optical modulator 104, aspheric optical lens 108 as anoptical lens to refract light from the aspheric concave mirror 107,polarizing beam splitter (PBS) 110 disposed between the reflectionspatial optical modulator 104 and aspheric concave mirror 107, and aquarter waveplate 105 disposed between the polarizing beam splitter 110and aspheric concave mirror 107. This collimating optical system is toproject light reflected at an arbitrary position on the reflectingsurface of the reflection spatial optical modulator 104 as parallelpencil groups from the optical lens 108.

The polarizing beam splitter 110 includes a polarizing selectivereflecting surface 103 formed from a polymer film as a polarized lightselector to allow P-polarized light to pass by and reflect S-polarizedlight.

The illumination light source 101, reflection spatial optical modulator104, aspheric concave mirror 107 and aspheric optical lens 108 aredisposed near or in close contact with four optical surfaces,respectively, of the polarizing beam splitter 101. Between theillumination light source 111 and polarizing beam splitter 110, there isprovided a polarization plate 102.

Illumination light projected from the illumination light source 101 isdetected by the polarization plate 102 to be an S-polarized light to thepolarizing selective reflecting surface 103 of the polarizing beamsplitter 110, and the majority of the light is reflected by thepolarizing selective reflecting surface 103. The illumination light thusreflected illuminates the reflection spatial optical modulator 104 whereit will be reflected in the incident polarizing direction kept as it isor in a direction resulted from rotation of the incident polarizingdirection through 90 deg.

In case the illuminated light has been reflected in the polarizingdirection kept as it is, it will be reflected again at the polarizingselective reflecting surface 103 and return to the illumination lightsource 101. On the other hand, the light of which the polarizingdirection has been rotated through 90 deg. and which has become aP-polarized light to the polarizing selective reflecting surface 103passes by the surface 103 and is reflected by the aspheric concavemirror 107.

In this case, the quarter waveplate 105 is provided between the asphericconcave mirror 107 and polarizing beam splitter 110 to rotate thepolarizing direction of the light reflected by the aspheric concavemirror 107 through 90 deg. and the light is incident as an S-polarizedlight again upon the polarizing selective reflecting surface 103. Thelight is thus reflected at this polarizing selective reflecting surface103. The light is projected from the polarizing beam splitter 110 andincident upon an optical waveguide 120 through the aspheric optical lens108.

The light incident upon the optical waveguide 120 is diffracted andreflected by the first reflection hologram element 123 for totalreflection inside the optical waveguide 120, and travels through theoptical waveguide 120 while being total-reflected. The light isdiffracted and reflected by the second reflection hologram element 124provided at the other end to depart from the condition of totalreflection, projected from the optical waveguide 120 for incidence uponthe pupil 16 of the viewer.

At this time, the divergent light projected from the reflection spatialoptical modulator 104 is formed by a combination of the aspheric concavemirror 107 and aspheric optical lens 108 into parallel pencil groupswhose curvature of field is very small.

The image display device 100 according to the present invention includesthe reflection spatial optical modulator 104, collimating optical systemto form the light reflected from the reflection spatial opticalmodulator 104 into parallel pencil groups, and waveguide optical systemto guide the group of parallel pencils projected from the collimatingoptical system by total reflection inside the waveguide optical system.By forming the collimating optical system from a combination of theaspheric concave mirror 107 as a reflecting optical element and theaspheric optical lens 108 as an optical lens, it is possible toimplement a collimator compact and of which the curvature of field isextremely small for an improved image resolution of the image displaydevice and a more compact and lightweight of the entire device.

Eighth Embodiment

FIG. 30 shows an image display device as an eighth embodiment of thepresent invention. The image display device is generally indicated witha reference numeral 130. The image display device 130 includes anillumination light source 101, image display transmission liquid crystalimage display element 134 as an image display element to display animage by making spatial modulation per pixel of illumination lightemitted from the illumination light source 101, and a virtual imageoptical system to receive illumination light spatial-modulated by thetransmission liquid crystal image display element 134 and guide it tothe pupil 16 of the viewer.

The transmission liquid crystal image display element 134 is for examplea transmission liquid crystal display or the like to make spatialmodulation of incident illumination light per pixel. Thespatial-modulated illumination light will be incident upon the virtualimage optical system.

The virtual image optical system includes a collimating optical system,optical waveguide 120, and first and second reflection hologram elements123 and 124 provided on the optical waveguide 120.

The collimating optical system includes a plane mirror 139 as a firstreflection optical element to reflect light projected from thetransmission liquid crystal image display element 134, aspheric concavemirror 107 as a second transmission optical element to re-reflect thelight reflected by the plane mirror 139, aspheric optical lens 108 as anoptical lens to refract the light reflected by the aspheric concavemirror 107, polarizing beam splitter 110 provided between the planemirror 139 and aspheric concave mirror 107, first quarter waveplate 135provided between the plane mirror 139 and polarizing beam splitter 110,and a second quarter waveplate 136 provided between the polarizing beamsplitter 110 and aspheric concave mirror 107. The collimating opticalsystem is to project light projected from an arbitrary position on theimage display surface of the transmission liquid crystal image displayelement 134 as a group of parallel pencils from the optical lens 108.

The transmission liquid crystal image display element 134, plane mirror139, aspheric concave mirror 107 and aspheric optical lens 108 aredisposed near or in close contact with four optical surfaces,respectively, of the polarizing beam splitter 134. An optical waveguide120 is provided between the transmission liquid crystal image displayelement 134 and polarizing beam splitter 110.

Illumination light emitted from the illumination light source 101illuminates the transmission liquid crystal image display element 134,the light projected from the transmission liquid crystal image displayelement 134 is detected by the polarization plate 102 to be anS-polarized light to the polarizing selective reflecting surface 103 ofthe polarizing beam splitter 110. The majority of the S-polarized lightis reflected by the polarizing selective reflecting surface 103.

The reflected illumination light is re-reflected by the plane mirror 139with which the first quarter waveplate 135 is in close contact, andincident again upon the polarizing selective reflecting surface 103. Atthis time, since the illumination light has been converted by the firstquarter waveplate 135 into the P-polarized light, it will pass by thepolarizing selective reflecting surface 103 and be reflected by theaspheric concave mirror 107.

In this case, the second quarter waveplate 136 is provided between theaspheric concave mirror 107 and polarizing beam splitter 110 to rotatethe polarizing direction of the light reflected by the aspheric concavemirror 107 through 90 deg. and the light is incident as an S-polarizedlight again upon the polarizing selective reflecting surface 103. Thelight is thus reflected at this polarizing selective reflecting surface103. The light is projected from the polarizing beam splitter 110 andincident upon an optical waveguide 120 through the aspheric optical lens108.

The light incident upon the optical waveguide 120 is diffracted andreflected by the first reflection hologram element 123 for totalreflection inside the optical waveguide 120, and travels through theoptical waveguide 120 while being total-reflected. The light isdiffracted and reflected by the second reflection hologram element 124provided at the other end to depart from the condition of totalreflection, projected from the optical waveguide 120 for incidence uponthe pupil 16 of the viewer.

At this time, the divergent light projected from the transmission liquidcrystal image display element 134 is formed by a combination of theaspheric concave mirror 107 and aspheric optical lens 108 into parallelpencil groups whose curvature of field is very small.

The image display device 130 according to the present invention includesthe transmission liquid crystal image display element 134 as an imagedisplay element, collimating optical system to form the light projectedfrom the transmission liquid crystal image display element 134 intoparallel pencil groups, and waveguide optical system to guide the groupof parallel pencils projected from the collimating optical system bytotal reflection inside waveguide optical system. By forming thecollimating optical system from a combination of the aspheric concavemirror 107 as a reflecting optical element and the aspheric optical lens108 as an optical lens, it is possible to implement a collimator compactand of which the curvature of field is extremely small for an improvedimage resolution of the image display device and a more compact andlightweight of the entire device.

Ninth Embodiment

FIG. 31 shows a virtual image display device as a ninth embodiment ofthe present invention. The virtual image display device is generallyindicated with a reference numeral 140. The virtual image display device140 includes an image display element 144 to display an image, and avirtual image optical system to guide incident display light displayedon the image display element 144 to a pupil 16 of the viewer.

The image display element 144 is for example an organic EL (ElectroLuminescence) display, inorganic EL display, liquid crystal display(LCD) or the like.

The virtual optical system includes a collimating optical system andwaveguide optical system. The waveguide optical system includes anoptical waveguide 150, reflecting mirror 153 provided at one end of theoptical waveguide 150, and a group of translucent mirrors 154 providedat the other end and parallel to each other. The virtual optical systemtotally reflects incident group of parallel pencils inside it, and thenprojects the group of parallel pencils thus total-reflected as it is tooutside.

The collimating optical system includes an aspheric concave mirror 107as a reflection optical element to reflect light from the image displayelement 144, aspheric optical lens 108 as an optical lens to refractlight from the aspheric concave mirror 107, polarizing beam splitter(PBS) 110 disposed between the image display element 144 and asphericconcave mirror 107, and a quarter waveplate 105 disposed between thepolarizing beam splitter 110 and aspheric concave mirror 107. Thiscollimating optical system is to project light coming from an arbitraryposition on the reflecting surface of the image display element 144 asparallel pencil groups from the optical lens 108.

The image display element 144, aspheric concave mirror 107 and asphericoptical lens 108 are disposed near or in close contact with threeoptical surfaces, respectively, of the polarizing beam splitter 110. Apolarization plate 102 is provided between the image display element 144and polarizing beam splitter 110.

The light projected from the image display element 144 is detected bythe polarizing plate 102 to be a P-polarized light to the polarizingselective reflecting surface 103 of the polarizing beam splitter 110.The majority of the P-polarized light is allowed by the polarizingselective reflecting surface 103 to pass by. The passing light isreflected by the aspheric concave mirror 107 with which the quarterwaveplate 105 is in close contact, and incident again upon thepolarizing selective reflecting surface 103. At this time, since thelight has been converted by the quarter waveplate 105 into anS-polarized light, so it is reflected by the polarizing selectivereflecting surface 103. The light is projected from the polarizing beamsplitter 110 and incident upon the optical waveguide 150 through theaspheric optical lens 108.

The light incident upon the optical waveguide 150 is reflected by thereflecting mirror 153 to be total-reflected inside the optical waveguide150, and travels while being total-reflected inside the opticalwaveguide 150. Then, it is reflected by the group of translucent mirrors154 provided at the other end and parallel to each other to depart fromthe condition of total reflection, projected from the optical waveguide150 and incident upon the pupil 16 of the viewer.

At this time, the divergent light projected from the image displayelement 144 is formed by the combination of the aspheric concave mirror107 and aspheric optical lens 108 into parallel pencil groups of whichthe curvature of field is very small.

The image display device 140 according to the present invention includesan image display element 144, collimating optical system to form thelight projected from the image display element 144 into parallel pencilgroups, and waveguide optical system to guide the group of parallelpencils projected from the collimating optical system by totalreflection inside waveguide optical system. By forming the collimatingoptical system from a combination of the aspheric concave mirror 107 asa reflecting optical element and the aspheric optical lens 108 as anoptical lens, it is possible to implement a collimator compact and ofwhich the curvature of field is extremely small for an improved imageresolution of the image display device and a more compact andlightweight of the entire device.

Tenth Embodiment

FIG. 32 shows an image display device as a tenth embodiment of thepresent invention. The image display device is generally indicated witha reference numeral 160. The image display device 160 includes an imagedisplay element 144 to display an image, and a virtual image opticalsystem to guide incident display light displayed on the image displayelement 144 to a pupil 16 of the viewer.

The virtual image optical system includes a collimating optical systemand waveguide optical system. The waveguide optical system includes anoptical waveguide 120, and first and second reflection hologram elements123 and 124 provided on the optical waveguide 120.

The collimating optical system includes an aspheric concave mirror 107as a reflection optical element to reflect light from the image displayelement 144, aspheric optical lens 108 as an optical lens to refractlight from the aspheric concave mirror 107, polarizing beam splitter 110disposed between the image display element 144 and aspheric concavemirror 107, and a quarter waveplate 105 disposed between the polarizingbeam splitter 110 and aspheric concave mirror 107. This collimatingoptical system is to project light projected from an arbitrary positionon the reflecting surface of the image display element 144 as parallelpencil groups from the optical lens 108.

The image display element 144, aspheric concave mirror 107 and asphericoptical lens 108 are disposed near or in close contact with threeoptical surfaces, respectively, of the polarizing beam splitter 110. Apolarization plate 102 is provided between the image display element 144and polarizing beam splitter 110.

The light projected from the image display element 144 is detected bythe polarizing plate 102 to be a P-polarized light to the polarizingselective reflecting surface 103 of the polarizing beam splitter 110.The majority of the P-polarized light is allowed by the polarizingselective reflecting surface 103 to pass by. The passing light isreflected by the aspheric concave mirror 107 with which the quarterwaveplate 105 is in close contact, and incident again upon thepolarizing selective reflecting surface 103. At this time, since thelight has been converted by the quarter waveplate 105 into anS-polarized light, so it is reflected by the polarizing selectivereflecting surface 103. The light is incident upon the optical waveguide120 through the aspheric optical lens 108.

The light incident upon the optical waveguide 120 is reflected by thefirst reflection hologram element 123 to be totally reflected inside theoptical waveguide 120, and travels while being total-reflected insidethe optical waveguide 120. Then, it is reflected by the secondreflection hologram element 124 provided at the other end to depart fromthe condition of total reflection, projected from the optical waveguide120 and incident upon the pupil 16 of the viewer.

At this time, the divergent light projected from the image displayelement 144 is formed by the combination of the aspheric concave mirror107 and aspheric optical lens 108 into parallel pencil groups of whichthe curvature of field is very small.

The image display device 160 according to the present invention includesan image display element 144, collimating optical system to form thelight projected from the image display element 144 into parallel pencilgroups, and waveguide optical system to guide the group of parallelpencils projected from the collimating optical system by totalreflection inside waveguide optical system. By forming the collimatingoptical system from a combination of the aspheric concave mirror 107 asa reflecting optical element and the aspheric optical lens 108 as anoptical lens, it is possible to implement a collimator compact and ofwhich the curvature of field is extremely small for an improved imageresolution of the image display device and a more compact andlightweight of the entire device.

Eleventh Embodiment

FIG. 33 shows an image display device as an eleventh embodiment of thepresent invention. The image display device is generally indicated witha reference numeral 170. The image display device 170 includes an imagedisplay element 144 to display an image, and a virtual image opticalsystem to guide incident display light displayed on the image displayelement 144 to a pupil 16 of the viewer.

The virtual image optical system includes a collimating optical systemand waveguide optical system. The waveguide optical system includes anoptical waveguide 150, reflecting mirror 153 provided at one end of theoptical waveguide 150, and a group of translucent mirrors 154 providedat the other end and parallel to each other. The virtual image opticalsystem totally reflects an incident group of parallel pencils inside itand ten projects it as it is to outside.

The collimating optical system includes a prism 180 having at least onetotal-reflecting surface. It is an optical system to project lightprojected from an arbitrary position on the image display surface of theimage display element 144 as parallel pencil groups.

The prism 180 includes a first optical surface 181 provided at thelight-incident side upon which the display light from the image displayelement 144 is incident and having no axis of rotational symmetry, asecond optical surface 182 provided at the light-outgoing sidecontributed to both the internal total reflection and refraction, and analuminum-made reflecting surface 183 as a third optical surface alsocontributed to the total reflection.

The pencil projected from the image display element 144 is incident uponthe prism 180 formed from the optical surfaces having no axis ofrotational symmetry first at the first optical surface 181. The pencilincident into the prism 180 is internally reflected at the secondoptical surface 182, then reflected at the aluminum-made reflectingsurface 183, and incident again upon the second optical surface 182. Atthis time, the incident pencil does not meet the condition of internaltotal reflection, and so it will be refracted and pass by the secondoptical surface 182 for incidence upon the optical waveguide 120.

The light incident upon the optical waveguide 150 is reflected by thereflecting mirror 153 to be total-reflected inside the optical waveguide150, and travels while being total-reflected inside the opticalwaveguide 150. Then, it is reflected by the group of translucent mirrors154 provided at the other end and parallel to each other to depart fromthe condition of total reflection, projected from the optical waveguide150 and incident upon the pupil 16 of the viewer.

At this time, the divergent light projected from the image displayelement 144 is formed by a combination of the second optical surface 182and aluminum-made reflecting surface 183 as reflecting surfaces and thefirst and second optical surfaces 181 and 182 as refracting surfacesinto parallel pencil groups whose curvature of field is extremely small.

The image display device 170 according to the present invention includesan image display element 144, collimating optical system to form thelight projected from the image display element 144 into parallel pencilgroups, and waveguide optical system to guide the group of parallelpencils projected from the collimating optical system by internal totalreflection. By forming the collimating optical system from a combinationof the aluminum-made reflecting surface 183 as a reflecting opticalelement and the first and second optical surfaces 181 and 182 as opticallenses, it is possible to implement a collimator compact and of whichthe curvature of field is extremely small for an improved imageresolution of the image display device and a more compact andlightweight of the entire device.

Also, in the image display device 170 according to the presentinvention, the collimating optical system can be designed furthershorter and more compact and the image display device 170 itself can bedesigned more compact by providing the prism 180 having at least atotal-reflecting surface in the collimating optical system.

Twelfth Embodiment

FIG. 34 shows an image display device as a twelfth embodiment of thepresent invention. The image display device is generally indicated witha reference numeral 190.

The image display device 190 includes an image display element 144 todisplay an image, and a virtual image optical system to guide incidentdisplay light displayed on the image display element 144 to a pupil 16of the viewer.

The virtual image optical system includes a collimating optical systemand waveguide optical system. The waveguide optical system includes anoptical waveguide 120, and first and second reflecting hologram elements123 and 124 provided on the optical waveguide 120.

The collimating optical system includes a prism 200 having at least onetotal-reflecting surface and an optical lens 191. It is an opticalsystem to project light projected from an arbitrary position on theimage display surface of the image display element 144 as parallelpencil groups.

The optical lens 191 has a first optical surface 192 provided at theprism 200 and a second optical surface 193 provided at the opticalwaveguide 120.

The prism 200 includes a first optical surface 201 provided at thelight-incident side upon which the display light from the image displayelement 144 is incident and having no axis of rotational symmetry, asecond optical surface 202 provided at the light-outgoing side andhaving no axis of rotational symmetry, and an aluminum-made reflectingsurface 203 as a third optical surface also contributed to the totalreflection.

The pencil projected from the image display element 144 is incident uponthe prism 200 formed from the optical surfaces having no axis ofrotational symmetry first at the first optical surface 201. The pencilincident into the prism 200 is reflected at the aluminum-made reflectingsurface 203, then refracted and pass by the second optical surface 202and incident upon the optical waveguide 120 through the first and secondoptical surfaces 192 and 193 of the optical lens 191.

The light incident upon the optical waveguide 120 is reflected by thefirst reflection hologram element 123 to be total-reflected inside theoptical waveguide 120, and travels while being total-reflected insidethe optical waveguide 120. Then, it is reflected by the secondreflection hologram element 124 provided at the other end to depart fromthe condition of total reflection, projected from the optical waveguide120 and incident upon the pupil 16 of the viewer.

At this time, the divergent light projected from the image displayelement 144 is formed by a combination of the aluminum-coated reflectingsurface 203 and first and second optical surfaces 201 and 202 asrefracting surfaces and the first and second optical surfaces 192 and193 of the optical lens 191 into parallel pencil groups whose curvatureof field is very small.

The image display device 190 according to the present invention includesan image display element 144, collimating optical system to form thelight projected from the image display element 144 into parallel pencilgroups, and waveguide optical system to guide the group of parallelpencils projected from the collimating optical system by internal totalreflection. By forming the collimating optical system from a combinationof the aluminum-made reflecting surface 203 as a reflecting opticalelement and the first and second optical surfaces 201 and 202 as opticallenses and optical lens 191, it is possible to implement a collimatorcompact and of which the curvature of field is extremely small for animproved image resolution of the image display device and a more compactand lightweight of the entire device.

Also, by providing the prism 200 having at least a total-reflectingsurface in the collimating optical system of the image display device190 according to the present invention, the collimating optical systemcan be designed further shorter and more compact and the image displaydevice 190 itself can be designed more compact.

Thirteenth Embodiment

FIG. 35 shows an image display device as a thirteenth embodiment of thepresent invention. The image display device is generally indicated witha reference numeral 210.

The image display device 210 includes an image display element 144 todisplay an image, and a virtual image optical system to guide incidentdisplay light displayed on the image display element 144 to a pupil 16of the viewer.

The virtual image optical system includes a collimating optical systemand waveguide optical system.

The waveguide optical system includes an optical waveguide 120, andfirst and second reflecting hologram elements 123 and 124 provided onthe optical waveguide 120.

The collimating optical system includes a triangular prism 220 having atleast one total-reflecting surface, first optical lens 211 disposedbetween the image display element 144 and triangular prism 220 and asecond optical lens 212 disposed between the triangular prism 220 andoptical waveguide 120. It is an optical system to project lightprojected from an arbitrary position on the image display surface of theimage display element 144 as parallel pencil groups.

The triangular prism 220 has a first optical surface 221 provided at thelight-incident side where the display light from the image displayelement 144 is incident, reflecting surface 222 as a second opticalsurface to totally reflect the pencil, and a third optical surface 223provided at the light-outgoing side.

The pencil projected from the image display element 144 is incident uponthe triangular prism 220 defined by planes first at the first opticalsurface 221 through the first optical lens 211. The pencil incident intothe triangular prism 220 is reflected at the aluminum-made reflectingsurface 222, then passes by the third optical surface 223, and isincident upon the optical waveguide 120 through the second optical lens212.

The pencil incident upon the optical waveguide 120 is reflected by thefirst reflection hologram element 123 to be totally reflected inside theoptical waveguide 120, and travels while being totally reflected insidethe optical waveguide 120. Then, the pencil is reflected by the secondreflection hologram element 124 provided at the other end to depart fromthe condition of total reflection, projected from the optical waveguide120, and incident upon the pupil 16 of the viewer.

The image display device 210 according to the present invention includesan image display element 144, collimating optical system to form thelight projected from the image display element 144 into parallel pencilgroups, and waveguide optical system to guide the group of parallelpencils projected from the collimating optical system by internal totalreflection. By forming the collimating optical system from a combinationof the reflecting surface 222 as a reflecting optical element and thefirst and third optical surfaces 221 and 223 as optical lenses and thefirst and second optical lenses 211 and 212, it is possible to implementa collimator compact and of which the curvature of field is extremelysmall for an improved image resolution of the image display device and amore compact and lightweight of the entire device.

Also, by providing the triangular prism 220 having at least atotal-reflecting surface in the collimating optical system of the imagedisplay device 210 according to the present invention, the collimatingoptical system can be designed further shorter and more compact and theimage display device 210 itself can be designed more compact.

Fourteenth Embodiment

FIG. 36 shows an image display device as a fourteenth embodiment of thepresent invention. The image display device is generally indicated witha reference numeral 230.

The image display device 230 includes an image display element 144 todisplay an image, and a virtual image optical system to guide incidentdisplay light displayed on the image display element 144 to a pupil 16of the viewer.

The virtual image optical system includes a collimating optical systemand waveguide optical system. The waveguide optical system includes anoptical waveguide 120, and first and second reflecting hologram elements123 and 124 provided on the optical waveguide 120.

The collimating optical system includes a plane mirror 235 to totallyreflect incident pencil, first and second optical lenses 231 and 232disposed between the image display element 144 and plane mirror 235 anda third optical lens 233 disposed between the plane mirror 235 andoptical waveguide 120.

The pencil projected from the image display element 144 passes by thefirst and second optical lenses 231 and 232, is reflected by the planemirror 235, and then incident upon the optical waveguide 120 through thethird optical lens 233.

The pencil incident upon the optical waveguide 120 is reflected by thefirst reflection hologram element 123 to be totally reflected inside theoptical waveguide 120, and travels while being totally reflected insidethe optical waveguide 120. Then, the pencil is reflected by the secondreflection hologram element 124 provided at the other end to depart fromthe condition of total reflection, projected from the optical waveguide120, and incident upon the pupil 16 of the viewer.

The image display device 230 according to the present invention includesan image display element 144, collimating optical system to form thelight projected from the image display element 144 into parallel pencilgroups, and waveguide optical system to guide the group of parallelpencils projected from the collimating optical system by internal totalreflection. By forming the collimating optical system from a combinationof the plane mirror 235 as a reflecting optical element and the first tothird optical surfaces 231, 232 and 233, it is possible to implement acollimator compact and of which the curvature of field is extremelysmall for an improved image resolution of the image display device and amore compact and lightweight of the entire device.

Also, by providing the plane mirror 235 having at least atotal-reflecting surface in the collimating optical system of the imagedisplay device 230 according to the present invention, the collimatingoptical system can be designed further shorter and more compact and theimage display device 230 itself can be designed more compact.

Fifteenth Embodiment

FIG. 37 shows an image display device as a fifteenth embodiment of thepresent invention. The image display device is generally indicated witha reference numeral 240.

The image display device 240 includes an image display element 144 todisplay an image, and a virtual image optical system to guide incidentdisplay light displayed on the image display element 144 to a pupil 16of the viewer.

The virtual image optical system includes a collimating optical systemand waveguide optical system. The waveguide optical system includes anoptical waveguide 120, and first and second reflecting hologram elements123 and 124 provided on the optical waveguide 120.

The collimating optical system includes a triangular prism 250 having atleast one total-reflecting surface, first optical lens 241 disposedbetween the image display element 144 and triangular prism 250 and asecond optical lens 242 disposed between the triangular prism 250 andoptical waveguide 120. It is an optical system to project lightprojected from an arbitrary position on the image display surface of theimage display element 144 as parallel pencil groups.

The triangular prism 250 has a first optical surface 251 provided at thelight-incident side where the display light from the image displayelement 144 is incident, second optical surface 252 provided at thelight-outgoing side and contributed to both the internal totalreflection and refraction, and a aluminum-coated reflecting surface 253as a third optical surface to totally reflect the pencil.

The pencil projected from the image display element 144 passes by thefirst optical lens 241, and is incident upon the triangular prism 250defined by planes first at the optical surface 251 through the firstoptical lens 241. The pencil incident into the triangular prism 250 isreflected at the second optical surface 252 as an internaltotal-reflecting surface, and then re-reflected by the aluminum-coatedreflecting surface 253. The reflected light is incident again upon thesecond optical surface 252. This time, however, the reflected light doesnot meet the condition of total reflection. So, it passes by the secondoptical surface 252 and then is incident upon the optical waveguide 120through the second optical lens 242.

The pencil incident upon the optical waveguide 120 is diffracted andreflected by the first reflection hologram element 123 to be totallyreflected inside the optical waveguide 120, and travels while beingtotally reflected inside the optical waveguide 120. Then, the pencil isreflected by the second reflection hologram element 124 provided at theother end to depart from the condition of total reflection, projectedfrom the optical waveguide 120, and incident upon the pupil 16 of theviewer.

The image display device 240 according to the present invention includesan image display element 144, collimating optical system to form thelight projected from the image display element 144 into parallel pencilgroups, and waveguide optical system to guide the group of parallelpencils projected from the collimating optical system by internal totalreflection. By forming the collimating optical system from a combinationof the aluminum-coated reflecting surface 253 as a reflecting opticalelement and the first and second optical surfaces 251 and 252 as opticallenses and the first and second optical lenses 241 and 242, it ispossible to implement a collimator compact and of which the curvature offield is extremely small for an improved image resolution of the imagedisplay device and a more compact and lightweight of the entire device.

Also, by providing the triangular prism 250 having at least atotal-reflecting surface in the collimating optical system of the imagedisplay device 240 according to the present invention, the collimatingoptical system can be designed further shorter and more compact and theimage display device 210 itself can be designed more compact.

In the foregoing, the present invention has been described in detailconcerning certain preferred embodiments thereof as examples withreference to the accompanying drawings. However, it should be understoodby those ordinarily skilled in the art that the present invention is notlimited to the embodiments but can be modified in various manners,constructed alternatively or embodied in various other forms withoutdeparting from the scope and spirit thereof as set forth and defined inthe appended claims.

What is claimed is:
 1. An image display device comprising: an imagedisplay element; an optical system; an optical waveguide having a lightincident port along a first portion of the optical waveguide and a lightoutgoing port along a second portion of the optical waveguide, theoptical waveguide configured to (a) receive light from the image displayelement through the light incident port via the optical system and (b)provide the light as output through the light outgoing port; a firstdiffraction grating positioned along the first portion of the opticalwaveguide; a second diffraction grating positioned along the secondportion of the optical waveguide; a light shield configured to preventleakage of the light through the first diffraction grating; and atransparent substrate having oppositely facing first and secondsurfaces, the first surface facing both the light incident port and thelight outgoing port, the second surface facing the light shield,wherein, the first portion and the second portion are at differentpositions along the optical waveguide, and the first diffraction gratingis between the light incident port of the optical waveguide and thelight shield.
 2. The image display device according to claim 1, furthercomprising: an air space (i) in-between a surface of the opticalwaveguide and the transparent substrate, and (ii) located on a side ofthe optical waveguide where the first diffraction grating and the secondare not present on the optical waveguide, wherein the transparentsubstrate is located on another side of the optical waveguide where thefirst diffraction grating and the second diffraction grating arepresent.
 3. The image display device according to claim 1, wherein theoptical waveguide is configured to reflect a portion of the light guidedthrough the optical waveguide a different number of times in the opticalwaveguide.
 4. The image display device according to claim 1, wherein thefirst diffraction grating or the second diffraction grating or both thefirst and second diffraction gratings have a plurality of interferencefringes.
 5. The image display device according to claim 1, wherein thefirst diffraction grating, the second diffraction grating, or both thefirst diffraction grating and the second diffraction grating have layersstacked together, each of the first and second diffraction gratingshaving an interference fringe.
 6. The image display device according toclaim 1, wherein an area of the second diffraction grating is largerthan an area of the first diffraction grating.
 7. The image displaydevice according to claim 1, wherein interference fringes of the firstand second diffraction grating are recorded with the same pitch.
 8. Ahead mounted display system comprising: an image display element; anoptical system; an optical waveguide having a light incident port alonga first portion of the optical waveguide and a light outgoing port alonga second portion of the optical waveguide, the optical waveguideconfigured to (a) receive light from the image display element via theoptical system and (b) provide the light as output through the lightoutgoing port; a first diffraction grating positioned along the firstportion of the optical waveguide; a second diffraction gratingpositioned along the second portion of the optical waveguide; alight-shield configured to prevent leakage of the light through thefirst diffraction grating; and a transparent substrate having oppositelyfacing first and second surfaces, the first surface facing both thelight incident port and the light outgoing port, the second surfacefacing the light shield, wherein, the first portion and the secondportion are at different positions along the optical waveguide, and thefirst diffraction grating is between the light incident port of theoptical waveguide and the light shield.
 9. The image display deviceaccording to claim 1, wherein the light shield is wider than the firstdiffraction grating.
 10. The image display device according to claim 1,wherein an edge of the light shield is between the first diffractiongrating and the second diffraction grating.
 11. The image display deviceaccording to claim 1, wherein the light shield and the seconddiffraction grating are non-overlapping.
 12. The image display deviceaccording to claim 1, wherein the light shield is disposed at a side ofthe transparent substrate where the first diffraction grating is notpresent.
 13. The head mounted display system according to claim 8,wherein the light shield is wider than the first diffraction grating.14. The head mounted display system according to claim 8, wherein anedge of the light shield is between the first diffraction grating andthe second diffraction grating.
 15. The head mounted display systemaccording to claim 8, wherein the light shield and the seconddiffraction grating are non-overlapping.
 16. The head mounted displaysystem according to claim 8, wherein the light shield is disposed at aside of the transparent substrate where the first diffraction grating isnot present.
 17. The image display device according to claim 1, whereinthe optical system is a collimating optical system that receives aplurality of pencils from the image display element and forms parallelpencil groups different in angle of field from each other.
 18. The headmounted display system according to claim 8, wherein the optical systemis a collimating optical system that receives a plurality of pencilsfrom the image display element and forms parallel pencil groupsdifferent in angle of field from each other.